Implantation bleeding?

Marissa
I was on birth control for three months, but a mix up with my prescription left me without it for two weeks. When the perscription came through, I decided to start using it mid cycle, thinking it would be better than waiting and be unprotected.
My period started (January 11) a few days before the pack's final week. I suffered intense cramps, fatigue, and a very heavy flow of dark brown and red blood. It lasted for about four days. After that, I had no problems.
Then, my boyfriend and I had sex on January 24th. Even though I'm on the pill we still use the pullout method.
Yesterday (January 28th) I was spotting brown blood. I also started feeling constipated, and cramps. 
I don't usually spot in between periods, so I'm worried it may be implantation bleeding. 
What do you think? 
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Last Period Started: January 11
Had Sex: January 24
Spotting: January 28