Had sex last night and my boyfriend ejaculated in me. The next day I started bleeding and cramping. Been off birth control for 3 days. Pregnant?

No. If you just stopped using birth control (and was using it correctly beforehand), then you should be safe. In order to get pregnant, you have to be near the time of ovulation. Typically that occrus 2 weeks after the onset of your period.
From being off BC. Your cramping and bleeding is most likely from stopping the pill, obviously you should see your gyn and he or she can do an exam, blood work and an ultrasound to rule out any other causes. If you are worried about becoming pregnant, you can still take over the counter morning after pill like Plan B up to 3 days after unprotected sex, and you can get a prescription pill Ella (ulipristal acetate) one up to 5 days after.