Learn Portuguese (Portugal): A constipação é um problema.

No, this is a very famous "false friend"! In Portuguese, uma constipação is a common head cold. If you need to talk about the digestive issue (constipation), the medical term is obstipação, but people usually say prisão de ventre.

How do we make constipação plural?

Like the vast majority of feminine nouns ending in -ção, you change the -ão to -ões. So, "the colds" would be as constipações. This is the most common plural ending for words ending in -ão.

Why is it um problema? Doesn't the -a at the end make it feminine?

Usually, yes! But problema is one of a few very common exceptions. Words that come from Greek and end in -ema or -oma (like problema, sistema, and poema) are actually masculine. Therefore, they take the masculine articles o and um.

If a cold is temporary, why does the sentence use é (the verb ser) instead of a temporary verb?

Even though a cold is temporary for the person who catches it, this sentence is making a general, universal statement about what a cold is. Whenever we define or classify a noun (saying "X is a Y"), we use the permanent verb ser.