I have an iud...And I've notice that afer having sex I get a bad smell. You wonder if its his sperm or would I have an infection?

Be seen. I would wonder if you have an infection or other problem, and you might need a culture.
DEPENDS. You might have an infection, usually bacterial which has a fishy or foul smelling odor. This odor can intensify after the introduction of sperm in the vagina after intercouse. The other possibility is that this is just related to intercourse especially if it resolves after a short period of time. As always, if unsure get it evaluated by uor doctor.