Pregnancy test!

So 3 weeks ago my boyfriend came inside me. I am not on birth control. Well my period was supposed to start 2 days ago so I've taken two pregnancy tests. One last night and one today.
The one from last night was negative but then when I looked at it this morning it was positive. Then my one today was negative. I'm pretty sad about it actually, because I was convincing myself I was pregnant. 
I know this could be an EVAP line but has this happened to anyone else and then ended up being pregnant? I may test again in 2 days. I'm also NEVER late. Haven't been late in years for no reason.