34 weeks pregnant been feeling nausea, dizzy, headaches feeling its hard to breath. Had a MRI done friday and they said my heart was a bit fast 120?

See OB/GYN. Asap you should see your ob/gyn. Could be a sign of multiple medical conditions that needs immediate attention.
No clear answer. I can't tie in an MRI to a incresased heart rate unless u might have been a bit anxious. The nausea/headaches etc are worrisome in pregnancy if tied to increasing blood preasure. Your OB needs to be aware of these symptoms & may want to screen you & check on the baby. Make the call.