I had unprotected sex 2 months ago, I took next day pill. After a week I had some bleeding (my periods used to be two weeks after sex). I have started to use birth control pills. A month after sex I've made blood HCG, it was <0,500. Now I have started 3rd package of pills, my bleeding during placebo days is not so intense as it used to be before using pills. Is it possible that I am pregnant or pills just make periods lighter?

It is most likely the pills. You're still getting periods but because the length of your cycle is different there could be less bleeding. As well the morning after pill will cause some irregularities in your periods for a few months after you take it. It could be a further reaction to that. So long as you're getting your periods, you should be fine.

Your HCG shows no pregnancy. As CravenMorhead said, the change in periods is from the next day pill and now birth control pills.