Since it improved using antibiotics it is probably not caused by a muscle spasm in the middle ear. Does your voice sound distorted to you? If so it may be Patulous Eustachian tube which is caused when the Eustachian tube is abnormally open.
The feeling of a "plugged" ear is usually due to a fluid build-up in the middle ear, caused by a cold, sinus, allergy, etc. The antibiotics will clear an infection and then the eustatian tube inflamation is reduced and the fluid can drain. The snapping or popping sound also can be from the eustatian tube blockage. I had fluid build-up that caused the popping sound and ringing in the ears. The ENT put a tube in a few weeks ago, and it is now much better. I also had the problem for 1 1/2 years before the Dr decided to try the ear tube. Good luck, hope you are feeling better soon.