От рюкзака у меня иногда болит затылок и шея.

A very literal breakdown is:

• От рюкзака = from the backpack / because of the backpack
• у меня = at me / for me
• иногда = sometimes
• болит = hurts / aches
• затылок и шея = the back of my head and my neck

So the structure is roughly:

Because of the backpack, for me, sometimes hurts the back of the head and the neck.

That sounds strange in English, but it is a very normal Russian way to express physical discomfort.

Yes, literally от рюкзака means from the backpack, but in this kind of sentence it means because of the backpack or due to the backpack.

Russian often uses от + genitive to show a physical cause or reaction:

• У меня от кофе болит желудок. = Coffee makes my stomach hurt.
• У неё от жары болит голова. = Her head hurts from the heat.

So here, от рюкзака means the backpack is the cause of the pain, probably from wearing or carrying it.

Because the preposition от requires the genitive.

The dictionary form is рюкзак.
After от, it becomes рюкзака.

A learner-friendly way to remember this is:

• от кого? чего? = from whom? from what?
• answer: от рюкзака

So this is just normal preposition + case grammar.

Russian usually expresses this idea with the body part as the subject:

• У меня болит голова. = My head hurts.
• У меня болит шея. = My neck hurts.

Literally, у меня means something like in my case / for me / at me, and the thing that actually hurts is the body part.

A few useful contrasts:

• У меня болит шея. = My neck hurts.
• Я болею. = I am ill / sick.
  This means being sick in general, not specifically that one body part aches.
• Мне больно. = It hurts / I am in pain.
  This focuses on the feeling of pain, not on naming the body part.

Мне болит is not the normal pattern here.

Because Russian usually does not repeat possessives with body parts when the owner is already clear.

Here, у меня already tells you whose neck and whose back of the head it is. So saying:

• у меня болит шея

already means my neck hurts.

Adding мой / моя would usually sound unnecessary unless you want special emphasis or contrast.

Затылок is the back of the head.

It is not the whole head, and it is not exactly the neck. In this sentence:

• затылок = the back of the head
• шея = the neck

So the speaker means pain in both areas, which makes sense if a backpack is heavy or sits badly on the shoulders.

This is a very common learner question.

Strictly speaking, there are two subjects:

• затылок
• шея

So you may expect plural болят.

And yes, болят is also possible:

• От рюкзака у меня иногда болят затылок и шея.

However, when the verb comes before a compound subject in Russian, singular agreement is often possible, especially in everyday speech. It can sound as if the whole phrase затылок и шея is being treated as one combined area of discomfort.

So:

• болит затылок и шея = natural colloquial wording
• болят затылок и шея = also correct, and a bit more formally “logical” because there are two subjects

If the subject came first, plural would be more expected:

• Затылок и шея иногда болят от рюкзака.

Yes, иногда can move.

In this sentence it comes after у меня:

• От рюкзака у меня иногда болит затылок и шея.

That sounds natural and neutral.

But Russian word order is flexible, so you can also say:

• Иногда у меня от рюкзака болит затылок и шея.
• У меня иногда от рюкзака болит затылок и шея.
• От рюкзака иногда у меня болит затылок и шея.

The basic meaning stays the same, but the focus shifts a little depending on what you want to emphasize first: sometimes, for me, or because of the backpack.

In normal context, it means the pain is caused by wearing/carrying the backpack.

Russian often leaves that idea unstated because it is obvious. A native speaker will naturally understand:

• the backpack is heavy,
• it sits badly,
• or it strains the neck and upper body.

If you want to be more explicit, you could say:

• От ношения рюкзака у меня иногда болит затылок и шея.
  = From carrying a backpack, the back of my head and neck sometimes hurt.

But the shorter original sentence is very natural.

Yes, but the nuance is a little different.

• от рюкзака often sounds very natural with physical effects on the body
• из-за рюкзака means because of the backpack in a broader, more general way

So both are possible, but от рюкзака fits especially well when talking about pain, bodily reactions, or discomfort.

Compare:

• От рюкзака болит шея. = very natural for physical strain
• Из-за рюкзака болит шея. = also understandable, but slightly less idiomatic in this exact bodily-symptom context

Болит is present tense of the imperfective verb болеть.

In this sentence, because of иногда (sometimes), it clearly describes a repeated or occasional problem, not something happening only once:

• sometimes my neck and the back of my head hurt because of the backpack

So the present tense here works like English present tense in general statements or habitual situations.