What could cause feeling of weakness in limbs. Haven't lost any strength but feels like they're getting weak. Vitamin deficiency?

Multiple causes. Dehydration, low blood pressure, and low blood sugar can cause a feeling of weakness. Also working out a lot, can increase need for protein. Look at life style and optimize, avoid tobacco smoke, alcohol, eat healthy unprocessed food with fresh vegetables and fruit and high quality protein, good oils, get a good night sleep, move through the day, try interval training. Reduce stress. Try mvmn.
Vitamin deficiency. Especially B12 could. You need to be seen and have labs done. Could be dehydration, diet issues, vitamin deficiency, or concerns like nerve issues. Again, you need to be seen and evaluated.