Conflicted

So i am confused. This boy and I have been talking since the end of February. A few weeks ago we started dating. I have had this class with a senior and i lowkey used to have a crush on him. Till recently we both spoke about it and he said the same too. Since then there have been some tension between us and last weekend he went to prom and went to parties. He hung out with this girl, and I have no right to be disappointed because i have a boyfriend but why do I feel this way? The senior and I joke around but to an extent it sorta hits me... whats up?!