34 weeks pregnant, discharged from hospital today. Hypertension, no protein in urine. Now having flashes in vision. What could that be?

Pre-eclampsia. This is a very serious issue. I suggest you see an ophthalmologist as soon as possible. If there is any indication for bleeding in the eye or swelling of the optic nerve, you should plan on delivery as soon as possible. The baby is now big enough for a healthy growth. This is urgent. Do not risk your health and vision!
See below. Sounds like is still pregnancy induced hypertension (pih). The important thing to distinguish it from is pre-eclampsia. Pre-eclampsia usually mandates delivery of the baby whereas pih can usually be managed until full term with bed rest and sometimes medications.