Nexplanon Arm Implant

Cassandra
I've had the Nexplanon Arm Implant since Nov 2015. I do not have periods on this form of birth control. I've had sex 4 times since October 2016 (I know, it's upsetting to me too ;) ) and after each time I get a "period" not even a week later. Initially I started freaking out thinking this was implantation bleeding, but pregnancy tests came up negative. 
This most recent time, May 25th - it was the same thing... had sex (with a condom) but I'm not sure of the integrity of the condom, things started to get a bit "messy/slippery" (sorry for the TMI) and not even a week later came my strange "after sex period". Since then, I've been eating everything in my path... feeling bloated and tired. Even my damn cats are up my butt trying to lay on my belly and I don't know why. Two days ago I had a small spotting. This part is not exactly normal for my "after sex period".
I'm just wondering if anyone has experienced similar things on birth control or if they think it could very well be likely I'm pregnant. I intend to take a test this weekend... I'm trying to put enough weeks in from the date of intercourse.