I was on sertraline 25mg for a month, then started 50mg. the other night i missed a dose. now my anxiety is terrible. is it because i missed one?

Not likely: Missing 1 dose won't do that. Talk to your doctor. You could be having a "paradoxical" reaction to the drug; or, "paradoxically," it could mean the drug is actually doing its job and giving you more (psychic) energy to experience your anxiety. If the latter, it may be a temporary effect as you "work through" the anxiety. Definitely let your doctor know.