It could be a urinary tract infection or pelvic imflammatory disease but your doing the right thing by going to the family planning clinic. You should also go to your GP first thing on Monday so he can take a urine sample. As for the Omeprazole, that has nothing to do with the symptoms, i take it that you suffer from acid reflux that is why you are using this medication? I use it too and the only side affects i have with this medication is headaches.
Do go to your GP on Monday and the clinic as you have booked it. In the mean time try drinking some cranberry juice and plenty of water. When you need to urinate try getting a jug full of warm water and pour over while you are urining, this will help ease any burning or soreness.
xx