Kulit lengan saya gatal, dan ada ruam kecil selepas saya bermain di atas rumput.

Because kulit means skin and lengan means arm. In Malay, nouns are often placed together to show a relationship:

• kulit lengan saya = the skin of my arm / the skin on my arm
• literally: skin arm my

So this phrase is more specific than just lengan saya, which would simply mean my arm.

Malay often leaves out the verb to be when describing something with an adjective.

So:

• Kulit lengan saya gatal = The skin on my arm is itchy
• literally: skin arm my itchy

This is completely normal in Malay. The adjective gatal works directly as the description, without needing a separate word for is.

Gatal means itchy or itching.

In medical or physical contexts, it describes the sensation of wanting to scratch something:

• Kulit saya gatal = My skin is itchy
• Mata saya gatal = My eyes are itchy

Be aware that gatal can also have other meanings in informal speech, including joking or negative social meanings, but in this sentence it clearly just means itchy.

In Malay, ada often means there is / there are / exists.

So:

• ada ruam kecil = there is a small rash / there are small rashes

The normal order is:

• ada
  • thing that exists

Examples:

• Ada masalah. = There is a problem.
• Ada ruam kecil. = There is a small rash / There are small rashes.

Putting ada after the noun would usually sound wrong in this kind of sentence.

In Malay, adjectives usually come after the noun.

So:

• ruam kecil = small rash
• literally: rash small

Other examples:

• rumah besar = big house
• baju merah = red shirt
• anak kecil = small child

This is one of the most important word-order differences between Malay and English.

It can depend on context. Malay nouns usually do not change form for singular or plural.

So ruam kecil could mean:

• a small rash
• small rashes

The surrounding context tells you which meaning is more natural. In English, the translation may choose singular or plural depending on what sounds best.

If Malay wants to make plurality very clear, it can use reduplication:

• ruam-ruam kecil = small rashes

But in normal speech, this is often unnecessary.

All of them can mean after, but they differ a little in style.

• selepas = common and neutral
• lepas = more casual, conversational
• setelah = a bit more formal or written

So in this sentence:

• selepas saya bermain = after I played / after playing

You could also hear:

• lepas saya bermain di atas rumput
• setelah saya bermain di atas rumput

All are understandable, though selepas is a very safe standard choice.

Bermain comes from the root main with the prefix ber-.

• main = play
• bermain = to play / be playing

In standard Malay, bermain is the usual form when talking about the activity of playing in general.

So:

• saya bermain = I played / I was playing

In casual speech, people may say main, but bermain is more standard and fits well in a full sentence like this one.

Yes, di atas literally means on top of / on.

So:

• di atas rumput = on the grass

Malay often uses:

• di = at/in/on
• di atas = on top of, on the surface of

Here it emphasizes being physically on the grass surface. In some situations, speakers might also simply say di rumput, depending on region and style, but di atas rumput is clear and understandable.

Because di here is a preposition, not a prefix.

• di atas = on top of
• di rumah = at home
• di sekolah = at school

When di means location, it is written separately.

But when di- is a passive verb prefix, it is attached to the verb:

• ditulis = written
• dibuka = opened

So:

• di atas rumput = correct
• not diatas rumput

The sentence has saya twice because each part has its own structure:

• Kulit lengan saya gatal
• selepas saya bermain di atas rumput

The first saya shows whose arm skin it is: my arm skin.
The second saya shows who did the playing: after I played.

Malay sometimes omits subjects when they are obvious, but keeping saya here makes the sentence clear and natural.

Yes. The given sentence is understandable, but there are a few natural alternatives depending on tone.

Some possibilities:

• Kulit lengan saya gatal dan timbul ruam kecil selepas saya bermain di atas rumput.
  This sounds a bit more natural because timbul ruam means a rash appeared.

• Kulit pada lengan saya terasa gatal, dan ada ruam kecil selepas saya bermain di atas rumput.
  terasa gatal emphasizes the sensation of itching.

• Selepas saya bermain di atas rumput, kulit lengan saya menjadi gatal dan muncul ruam kecil.
  This sounds a bit more carefully structured.

But the original sentence is still perfectly understandable for a learner.

No. Malay does not normally use articles like English a, an, or the.

So:

• kulit lengan saya can mean the skin on my arm
• ruam kecil can mean a small rash or small rashes
• rumput can mean grass or the grass

English needs articles, but Malay usually leaves them out and relies on context.