Weird Period

My period this week was really weird where I bled normally on the first day (medium flow) and then I didnt bleed for two days, just light spotting or nothing (which is weird because my second days are usually my heaviest) and then today I bled a light amount but it looked about the amount of a light period. However, the blood was brown, which makes me wonder if its implantation bleeding?

I did have unprotected sex the week before (he pulled out though and we didnt go any further after he came). So I took a pregnancy test and it looks pretty negative. Anyone have any explanation for these weird period events?

Here's the pregnancy test