I Had Sex With My Boyfriend On Monday April 4th

I had sex with my boyfriend on Monday April 4th and we used a condom. My periods have been very irregular since I stopped the Depo shot in May of 2015. My last period started on March 18th and the one before ended on the 4th of March. I had some cramping and vaginal discharge on that Tuesday through Wednesday and then Thursday I had very mild spotting. I have felt tired and since before sex on Monday I felt nauseous. I have been under a lot of stress this week from schooling and I can't tell if the spotting is from stress or from implantation bleeding. What is the probability I might be pregnant?