Boyfriend “isn’t in the mood”

My boyfriend and I normally have sex every other day or every other second day and have been together for six months now. It has now been 6 days since he has even made an effort to engage in sex and has denied all my efforts on a claim that he is “too tired” or “isn’t in the mood”. I am 20, he is 21. We both work part time and have been off the last three days and are currently not attending summer school so I’m not sure what would have him sooo exhausted. I’m writing this post because in all honestly I feel unwanted and undesired by him and it hurts. Six days isn’t a crazy long time and I’m aware of that but it just isn’t normal for us. Has this happened to anyone else?