Spotting! Am I pregnant

my period is very irregular so I’m never sure when it comes. so i had unprotected sex on the 3 and 4th of this month , I took a pregnancy test 4 days ago and it was negative. the last 2 days my breast and back have been hurting bad! Yesterday my back was hurting really really bad . When I woke up today I wiped myself and I had a brown to reddish discharge . Never have I had it , it had continued all day today it looks mostly brown . What should I do ? Could I be pregnant? Should I take another test ?