Hmm...well, this might not be applicable at all, as I wouldn't expect much of a...er..."set up," so to speak, upon starting, but are you using any lubrication or anything else? Don't have to get into details in respect for your privacy, but I'm wondering if you're having an allergic reaction to something.

Otherwise, I don't think tensing up would cause nausea...granted, musculature knowledge isn't my forte, but that's my guess. Dr. Skipper and George brought up good points...are any of those applicable? And Hamster brings up a great point. Hydration in pretty much anything is vital.

That's the best I could come up with after racking my brain. I would certainly advise talking to a doctor, as they can help you more than any of us can, try as we may.

I wish you all my best, and I hope things improve.

Hugs,
Harley