@Coco2 - integrate bp with that and time the positions for a good test.
Your heart rate (hr) and blood pressure (bp) are supposed to have a rise when you stand. It should not stay flat - that would be a problem. Likely a pretty bad problem. So there should be a rise but not much of a rise and it should stay elevated for a short duration but not too long of a duration.
The Doc can easily do a hr and bp test that does involve you lying down, then sitting then standing. It is timed in how many minutes this position and that position. Gives an idea. They can do this right in the office - no special equipment needed and no special training. Nurse, Doc, they all should be able to do this without blinking an eye.
The ECG (electrocardiogram) which you will be getting shows a short duration (maybe 30 seconds or a couple minutes) of electrical activity of your heart. If nothing of interest shows up, you could ask your Doc whether a Holter Monitor might provide more information. A Holter Monitor is quite similar to an ECG with the exception of the Holter Monitor recording for a longer duration. 48 hours is typical although the Doc can order whatever time duration they think would be appropriate. You wear it and go about your life. If you feel any “episodes” then you press a button and it places a “marker” on the data stream so when the Doc later reviews the data, they can see if any electrical activity of note was also occurring at that time. IMHO this is a nice test.
You could also ask your Doc if an echocardiogram (ultrasound of the heart) would be appropriate. This is also very non-intrusive and can be very helpful.
Note that ECG very specifically refers to electrocardiogram and NOT echocardiogram even though you might think it could go either way.
Whether these tests show anything or whether they show nothing - that all is great information to help your Doc to either come up with a diagnosis based on a result or rule many serious issues out of the picture based on no results.