My first thought was also POTS. You can do a general test for it yourself, it's not diagnostic but will indicate whether or not it's worth pursuing a referral to Cardiology.
Lie down for 5-10 minutes. Nice and calm and relaxed, no speaking! Then take your pulse to work out your heart rate (beats per minute - BPM). Then stand up. Stand still, no jiggling or moving around. After 10 minutes take your pulse again to calculate your BPM.
Then work out the difference between the two. If your standing heart rate is more than 20 BPM more than your resting heart rate that indicates some form of dysautonomia, if it's more 30 BPM more that indicates POTS. Only a tilt table test is completely diagnostic but my results doing this were very similar to my tilt test.
It's best to do the test early in the morning as orthostatic intolerance can get better or even completely resolve over the course of the day, but that doesn't mean you don't have POTS. I also wouldn't recommend doing it if you've had alcohol the night before. It might not matter for people without POTS but any amount of alcohol sends my orthostatic intolerance into the stratosphere, even the next morning. So it can really impact results.
Another thing, do NOT rely on a Fitbit or similar to measure your heart rate. They're fine when your heart rate is fairly stable but can't keep up with the rapid changes that happen with POTS upon standing. If you have a chest band monitor then use that but if not just manually take your pulse to calculate BPM. If I stand up my heart rate can instantly shoot up to 130 or thereabouts from a resting heart rate of 55 BPM, which is accurately captured by my chest monitor, but my Fitbit will be happily sat on my wrist telling my heart rate is only 77 BPM.
If your test indicates that POTS/dysautonomia is a possibility then get yourself back to your GP and ask for a Cardiology referral. In my area the waiting time is 17 months so you should really get yourself on the list ASAP if needed. I actually ended up going private as I was so unwell with it. I finally get to see my cardiologist on the NHS in December (same cardiologist!), at which point he'll probably be ready to discharge me but at least I should get my medication on the NHS.
POTS isn't curable but there's a lot you can do to manage it. Medication can help but won't be enough on it's own, the others things are physiotherapy (huge help for me, including following the Levine Protocol), and lifestyle changes such as pacing, as well as increasing salt intake and wearing compression stockings. I've been incredibly unwell with it but the sooner you get a diagnosis the better.
Good luck!