Since the condom was still on the mans penis, even if it HAD slipped, as long as it was covering the head of his penis, you had (have) nothing to worry about.
That you got ill 18 days later I don't believe has anything to do with the sex you engaged in. I fear you ARE overacting, and I don't believe you need to test, but regardless of that, it is your right to test if that's what YOU need for peace of mind. Go ahead and play safe. It's your body, your life.
I wish you the best
RubyWitch