Hello,
I've been having unregular menstration cycle ever since I first got my period which was when I was 13.. Iam now 22 and recently I didn't get my period for 3 months-this has happened before-i wasn't worried if I was pregnant cause at the time I was single and wasn't having no sexual relationship with no one.. but now I am with someone and my period has been coming regularly-every month (iam kind of shocked)-but the last time I got it which was on the 16 of August I only had it for 4 days it didn't last 6 days like usual.. could this unregularity be a sign of me being steril?

No this is not a sign of being sterile I was never regular, sometimes I would go for months and not have a period. Guess what? I have 3 beautiful children

No, this is actually very common. If you go on birth control pills, you will become more regular.