What could it be? Paranoid.

So on 3/19 I had sex with my so and he took off the condom and withdrew, he did not cum at all that day, and my period is supposed to start Monday-ish (my period is irregular) recently for the past 3/4 days I've been having dark brown spotting and some cramps. I looked it up and seen it could be implantation, but it's been over 3 weeks?  I've been super paranoid and  not sure what it is. It's really freaking out so I need advice. Has anyone experienced this? Anything would help!