What does a lower heartbeat while on my back mean?

[Panthira]

This pregnancy has just sucked for me physically. It's 3:30 AM and I'm up with a screaming headache. It seems like I have headaches every other day, but I was getting them pretty often without being pregnant too.
I've been feeling the sensation of knives on what I think is my cervix since very early in the pregnancy. My midwife tells me that it's early for feeling this sensation and it usually means your body is gearing up for labor and the baby is putting pressure right there.
I can't walk very far or the pressure gets really bad and I start to feel crampy. I'm 34 weeks on Saturday but have felt this way for many weeks. I've put myself on partial bedrest since week 28, and this has really helped.
My mood swings are really out of control.
There are just so many other aches, pains, and issues I've had and have with this pregnancy, and though I know it could be so much worse I'm not enjoying being pregnant at all.

Maybe I'm just getting too old to have babies. I'll be 36 in January. I worry a lot about the baby and especially realize that bad things CAN happen to me.

One thing I've noticed is when I lay on my back and sometimes my left side, I feel like I'm not getting enough oxygen or it doesn't feel right, like something is being cut-off. When I went to my last couple of appointments and they checked the heartrate of the baby, his heartrate would drop quite a bit if I lay on my back during the check. What could this mean?
Also, my sitting heartrate is pretty fast, sometimes I wake up with my heart just racing.

Thanks for reading my middle of the night worry ramble. :shy

[StarCat]

when you lay on your back later in pregnancy the weight of the baby can put pressure on the inferior vena cava which brings blood from the lower body back to the heart. This causes a drop in your blood pressure, increase in heart rate, nausea, anxiety, etc. It also causes a decrease in blood getting to baby so baby can drop his heartrate, too. Its temporary and easily corrected (just change positions) Some women experience it really badly- some women it doesn't bother at all. Depends on baby's position and probably mom's anatomy, too.

Your body has done wonderfully growing this baby so far and will do perfectly the rest of the journey! I have been having baby worries, too. I think its normal for where we are at in pregnancy. Pretty soon we will have those babes in arms and all will be well. Just try to keep telling yourself this is what you were made for. :hug