Exactly the same thing happened to me last year when I was 19!
I went to my doctor at home, she diagnosed sciatica (sciatic nerve pain which occurs in the lower bag and can go down your leg), probably caused by a disc, but she didn't take it seriously and just said to take paracetamol and it would go away by itself!
A couple of months later with constant pain, I went to the Philippines to go to a funeral but while I was there, I saw a doctor who made me get an xray and MRI which confirmed I had a slipped disc and two disc bulges.
After I returned to the UK, I saw my GP at uni, showed him the xray and MRI scans, and then he referred me to an orthopaedic centre for assessment.
Long story short, after 8 months of horrific pain and a long referral process, I got referred for 6 weeks of physio from an orthopaedic specialist. And another 4 months later (after not really doing much of the physio they told me to do) the pain has eased little by little, to the point where I get only small twinges now and again. An MRI in December shows that now I only have a small disc bulge and no slipped disc.
My advice to you is:
Go back to your GP. Make sure they understand how painful your sciatica is. Make sure they do a 'straight leg test' on you - as this will help them confirm it (all GP's and specialists should do this test to assess the problem).
Make sure to ask your GP for a referral. Tell them you want to get an MRI and get treated. The first course of action is physio, if that doesn't help then probably steroid injections. But make sure you keep nagging them about a referral even if they put you off. The sooner you start physio, the better.
Also, one last tip: as much as it hurts, try and keep active. Even walking for twenty minutes a day will help.
Take breaks between sitting down for long periods. If you're having trouble sleeping with the pain, I've found that sleeping on my side, with my other leg bent, really helps relieve pressure on the sciatic nerve.
PM me if you have any questions