Random nauseous feeling when lifting

Kyra
While I was at work the other day I got a random nauseous feeling when I lifted a bag of bakery mix(it was only maybe 30 lbs)
I don't want to let my mind wander, but my fiancé and I have been ttc since January and just started using pre-seed 2 months ago. 
I've never had the nauseous feeling happen before. I am used to lifting heavy items and have been doing it for almost 6 years at my job.
What could it be?