Lightheadedness/pass out feeling while performing HIIT?

I feel like I've been eating right and properly hydrating throughout the day, but sometimes I get lightheaded enough that I feel like I could pass out if I continue working out at a decent pace… I also feel like I am exhausted even though I am not breathing heavy… When I check my heart pulse during this time of light headedness it is low in the 110-120 zone compared to my average of 160-170 and I can feel the actual pressure seems low as well… I know I cant tell the exact pressure for sure, but it definitely isn’t throbbing as much as other times I feel fine and check it during my HIIT workouts…

Today my workout consisted of: 21-18-15-12-9-6-3 reps as fast as possible of: push ups, squats (no weight), dumbbell curls, flat leg sit ups, and shoulder press… Right around the 12 minute mark I started feeling lightheaded like I described above… My time was a horrible 23 minutes when last week I did this workout in 18 minutes… Last week I felt fine throughout…

I have noticed this phenomenon to be intermittent… Sometimes it’s a week or two before it happens… Sometimes its shorter/longer… I used to think it was from drinking over the weekend, but I have cut back a lot and it still persists… Could this be a hormone problem or something physiological I should get checked out? I am 31 now and haven’t had a physical/checkup since I was 20 in the Army… Any suggestions would be appreciated…