What Causes Spotting With Lower Back And Abdominal Pain?

i wiped on Monday and had pink blood and on Tues nothing and Wed this week so far nothing my period isnt due till Sat/Sun. I have been trying to conceive is it best to do a pregnancy test now or wait till Sat/Sun and why would I have a bit of blood on Mon a week before period due. I have lower back pain and slight dull abdominal pain - just like menstrual pain but not as severe as I suffer quite bad menstrual pain on the first couple of days