Please excuse my tardy response. I'm sorry to learn about your pain. Congrats on becoming a mom for the 2nd time.
An "RA" test primarily rules out rheumatoid arthritis. There are over a 100 different types of this painful disease. People in their 20s and 30s can get osteoarthritis OA, although there is often an underlying reason, such as joint injury or repetitive joint stress from overuse. If you fit that description OA may be a factor in your pain.
However I tend to think your PCP may be on the right track. I would be sure my vitamin and mineral levels are checked, especially the Vit D and B12, Iron and Calcium levels. If indeed they are low a one week supplement will not bring your levels up significantly. It should also be determined why any levels are low.
An OB/GYN may have some answers as this seems to be pregnancy or postpartum related.
If all the nutritional and hormonal possibilities are been ruled out I'd than consult a rheumatologist, They don't just treat arthritis - they treat most all inflammatory conditions.
Pain/discomfort is your bodys was of signalling you that something is wrong. We should always listen and take heed. I wish you the best and hope you'll let us know how you are doing.