Increased heart rate, back pain, and hard belly

Hi there

I've only been on this site for 5 minutes, but I already have a question...
Here it goes:

I've been dealing with terrible, aching lower back pain for 3 days. I thought these were engagement contractions because our baby (I'm currently 36 weeks) hadn't engaged 2 weeks ago. This came with a lot of hard contractions, very irregular, but sometimes painful. As soon as I stood up, sat down, moved slightly, or just moved at all, I felt enormous pressure on my belly and it became hard (I assume these are the hard contractions).

Now I've also had fluctuating blood pressure for 2 days, but mainly my heart rate shoots up enormously, even at rest. Yesterday my heart rate went way above 110, heading towards 120, and my blood pressure was already 15 over 9 within 5 minutes. It would drop, then go up again... It feels exactly like your heart is jumping out of your chest and you're very short of breath.

We were told to go straight to the maternity ward to check everything out just to be safe, but once there, everything was back to normal. Blood pressure went down, heart rate also after a while, and the baby sounded okay too. That same afternoon I also felt enormously that the baby was pushing downwards (hips/pubis), and every movement 'it' makes becomes more painful.

Has anyone experienced this before?

We have the feeling that it will come sooner rather than later, that the OB might have made a mistake (just like with my sister-in-law, he was fully term but supposedly 3 weeks early), because we also have no idea when conception actually took place (I'm very irregular).
I wonder then, how do you know it's almost time?

ps: Tuesday we have another check-up with the OB

Best wishes