It could be several things actually.

1. If you're taking antidepressants or something like that, they can case delayed orgasm or anorgasmia entirely. I'm still recovering myself from that problem.

2. Performance anxiety can also cause delayed ejaculation because your subconsciously holding back. (This is what I have now.) You probably won't notice it unless you get really close to feeling how your body is reacting.

3. Masturbation can cause delayed ejaculation. I read that it can be good for the prostate, but too much and you can desensitize yourself making it harder to reach orgasm.

Personally I would suggest first talking to your regular doctor and if they find everything physically fine then consider talking to a therapist. Could be any number of things, but there's a solution for all of them.