Can implantation bleeding occur within only 3 days after conception?

Me and my boyfriend had sex June 25th. I'm on birth control, but I missed a pill 2 days before engaging in intercourse, and the day after. I also don't take my pills at the same time every day. We only had sex 3-4 days ago and now I'm having cramps and a very small amount of brown spotting. Its definitely not period blood, as it is extremely light. My period also isn't supposed to start until June 3rd. I know implantation bleeding doesn't usually occur this early, but is it possible? Could I be pregnant?