Elevated heart rate

So I'm 33 weeks pregnant and earlier today I was literally just sitting down and felt my pulse skyrocket. It lasted maybe 4 minutes ? Not quite sure what it meant but it made me feel very lightheaded and I wasn't even doing any physical activity ! Was this my body's reaction to a contraction that I didn't feel? I've been drinking water by the gallon...daily. So I ruled out dehydration on my own lol. Is this normal ? I have been having some BH contractions on/off for over a week so I'm hoping today's incident was just another set of them.