Should I go back to the hospital

Got diagnosed with a UTI the other day without urine sample or anything just because "it sounds like what I had before" earlier in my pregnancy. 
I got put on Nitrofurantoin and have now been taking it for 3 days. Yesterday and today the stomach and lower back pains have got worse and I've found myself today almost fainting and having to drink 6 litres of water yet still not clenching my thirst. 
I can barely move and although I've checked baby on my Doppler and her heartbeats fine, I still feel worried.
Do I leave it another day or two or head up now?