Pregnant?! Or something else?

Hello! So December 5th was possibly the first day of my last period. I say it like that because I had unprotected sex on the 15th of December and then I started bleeding on the 29th, which what I thought was my period, but maybe implantation bleeding? I think that because it would be only a 24 day cycle and I'm usually always 28 days. Also, it was weird ..heavy the first day then lighter and only lasted 3 days. Which my periods usually last 5.

I was curious after that so on jan 2nd I took a pregnancy test & after 5 minutes it was negative but I put it away because I was going to give it more time before I looked again. Well, I never looked at it again because I forgot. I guess I wasn't that concerned.

So that bring me to now. If the bleeding that started on the 29th was my period then I should have started this months on the 26th, 3 days ago.

I noticed the calendar this morning so I was thinking about my weird period last month and remembered that pregnancy test i put away. I looked at it again today and its a very faint positive. Immediately I went and bought more pregnancy tests, did 2 of them and both negative...

WTH is going on?!?!

Now another little side note, I know stress can affect menstrual cycle & I have been very stressed for the last couple months. So maybe that's a factor? Idk

OH! & I have no other symptoms of pregnancy

Anyone have this happen or know what is going on?

P.s. I'm 21 years old, never been pregnant before, & I have regular 28 day cycles & not on birth control....