nexplanon pain

I only get my period every once and awhile but when i do it’s the beginning of the month for a week but last month i ended my period and had unprotected sex right after i started to spot brown blood but i didn’t think much of it. than the next day i had sex again and i started to spot pink blood and i spotted for like 4 days straight after that. today i woke up with horrible stomach pain so i took a test and it came back negative. idk what’s going on with me but i’m in so much pain.