Why do legs hurt if a person sits for a long time?

Hello doctor,

I am a 20-year-old female, and I live a healthy lifestyle. However, my legs and back are always aching. It has gotten progressively worse over the last few years, and it is beginning to affect my quality of life. When the pain is intense, I get nauseous and throw up from the pressure it puts on my abdomen. I have found sitting for extended periods aggravates it. For example, this past weekend, I went on a trip with friends that required sitting for a long car drive. I was throwing up for 30 minutes of the ride and felt sick for the remaining trip. In addition to this, I have been experiencing chronic fatigue, muscle spasms, and urgency in bowel movements, and I get severe hunger pangs even shortly after I eat. I do not know if any of these things are correlated or not. My stress is managed; I get eight hours of sleep each night, go on daily walks, and eat a healthy diet. I had an ultrasound of my gallbladder and kidneys, and no significant findings were found. I am at university, and now that school has started, it is hard to sit in a chair for extended periods to work on homework or attend class when my body is aching. I am constantly nauseous, hungry, and fatigued. I take the tablet Tylenol almost daily to relieve the pain, but with no relief. I thought I would reach out and see if anybody might know what I am experiencing or if there is anything I can do to help it.