This sounds like orthostatic intolerance (inability of the body to adjust blood circulation / flow when upright).
Ask your doctor to check your orthostatic vitals. You can also do this yourself at home if you have a way to measure your blood pressure and heart rate. (Search “NASA lean test” or “active stand test” for instructions.) Be mindful that you might become faint during the test – have someone else present or position yourself near a bed or soft chair in case your legs give way.
It’s also a good idea to keep a log of your symptoms, activity, and blood pressure and heart rate.
There are quite a few possible causes of orthostatic intolerance. Ask your doctor to check you for them. If they rule out vitamin/mineral deficiencies, medication side effects, endocrine causes, heart problems etc. and you are still unwell, you can ask to be referred to a long COVID clinic, a fainting clinic, or an autonomic specialist.
Fainting “for no good reason” needs to be investigated. Meaning, if there is no obvious cause for you to faint, and you are feeling generally unwell between fainting episodes, then it needs investigating.
So your doctor needs to recognise that this is chronic and ongoing, rather than being isolated fainting episodes that can be explained by circumstances between which the patient feels normal.
(I have chronic orthostatic intolerance so that’s why I know a bit about it. In my case it is due to autonomic dysfunction, but there are other causes.)
Best of luck getting some help.