Wanting but not wanting sex?

Tallulah
Ive had sex with my boyfriend countless times and its never not good, sometimes its not as up to scratch as usual but most of the time it is pretty damn good.
The other day though we had sex and it was really good and then suddenly it wasnt and I cant get aroused enough for it to be comfortable and now I have no sexual apetite whatsoever. I still find my SO very attractive and damn sexy but no drive? Its been nearly 3 weeks, has anyone experienced this or have any clue what might be going on??