Period 4 days late, sharp pelvic and rectal pain, mild spotting, shoulder pain, some dizziness, pulse/heartbeat on right side like a quiver?

See your doctor: Several possibilities, including pregnancy or pregnancy problems such as a tubal/ectopic (out of place) pregnancy than could be serious. Might also be other problems such as hormonal imbalance (even polycystic ovarian syndrome--pcos), stds, other infection. Best to check it out. If you don't want to be pregnant, you should discuss birth control. If you do, you should talk about that.

Go to ER: Short answer is, if you are pregnant, it could be in the wrong (/ectopic) location, which can cause symptoms as you describe, and deteriorate into a life-threatening condition. This is unlikely, but should be safe and be checked asap--also may find alternate diagnosis while being evaluated.