I let my virginity a while ago. I got my period as normal, and haven't had sex since because I was never in another long term commuted relationship. Beginning of January I was, so we had sex. I got my period 20 days later (on time) but it was a lot lighter than normal and only lasted for 3ish days and the 4th was just old blood. Normally it's a medium bleeding for 5 days. I don't know if having sex can change your cycle or hormones?

Some women miss an entire month, some have extra bleeding in a month,

But what you described is also a sign of pregnancy in some women, so you wait till next month and see if you have a period.

But I tested negative.