Implantation Bleeding vs. Menstruation

Hey everyone! I've been off of the pill for 4 months now. My periods were right back on track with 28 day cycles. My last period was on Dec. 27th and then I had brown spotting for 4 days starting Jan. 12th. 2 weks later I notice that my period is late and Jan. 31st I have light bleeding on and off; as well as today too. Ive taken 2 tests and both were negative. I had nausea, constipation, bloating, & cramping (not intense). I've never had a period like this before so im at a loss as to what's occuring and want to know if anyone has been through this? Thank you:-) p.s. yes I've had intercourse multiple times in January.