I'm having 3 days period for roughly 5 months now but I used to have a normal period of 5 days.. I'm getting worried about this.. is it that I am pregnant or is this normal?

How old are you? It's possible your periods are changing due to age and hormones.

I'm 18..

Have your activities or stress levels changed? How about diet and exercise? Weight loss or weight gain?

Are you taking any birth control?

Typically the first sign of pregnancy is a missing period.

I am quite fine nothing much changed in my daily routine.

Hat form of birth control do you use?

No none at all

So you are trying to get pregnant then? Correct?

No and I'm just asking why my period became 3 days period instead of 5 days

It could be due to pregnancy since you aren't doing anything to prevent it, but that is remote.

I suggest you test using first morning urine to be sure.