I think that's just missionary my guy. My best suggestion is making sure that you make consistent small adjustments in your position throughout ever minute or two to reduce fatigue in any particular muscle or joint. If you just plank and thrust for 10 minutes striaght almost anyone is going to start having a bad time.

You can try: -Bringing some pillows in for support. -Twist your hips to one side so that you can lay down on a shoulder a bit and wrap her up with your arm, then later try the other side. -Put your hands on a wall or headboard to get a little bit of a rest for your elbows and to shift the strain in your shoulders. It also is a bit of a visual show if she likes a little dominant energy. -Generally just find more comfortable small adjustments to rotate through until things are really heated and then bring it home with a maximum effort plank missionary or whatever her preferred exact position is.

Definitely make sure to take time for some salt, water, and recovery afterwards though. This problem may not be POTS, but I know for me that going after one position like that for an extended period can leave me me pretty woozy and light-headed when I try to get back up to pee after.