P.S. I missed:
“I would like to request that my doctor test the fluid in the joint but I’m wondering if anyone knows how painful this is?”
Get this done by the expert rheumatologist that I mentioned. Never ever get a joint fluid test from a doctor who does not have immediate test facilities. Also, check that those facilities include all appropriate tests, not just a polarizing microscope.
If fluid cannot be tested as soon as it is drawn, it is a complete waste of time.
If tests do not include all life-limiting possibilities, you put your health at serious risk.
Finally, if it is done correctly, you will not feel anything at all. Trust me, as I’ve had my ankle done by a properly trained rheumatologist. I had to ask when the test was going to start after it had already been done! But, a less qualified medic might not have the right quality of needle to ensure a painless fluid draw.