Today I tried doing bicep curls since highschool and my elbow started hurting really bad like it was going to break. It was only 5 pounds. Whats wrong?

Get checked. That kind of severe pain without clear injury can suggest something is out of place (dislocated), or something is caught where it shouldn't be, such as a ligament inside the joint being pinched. Both unusual without some injury though. Need more info to make clear diagnosis as fractures, infections, inflammation (bursitis, tendinitis) can also cause pain.
See your doctor. Could be a number of things, like bicep tendon rupture, bad tennis elbow, etc. An x-ray and physical exam will help.