Hi there. Sorry to hear that. I know how upsetting it is when you think you are getting somewhere and then something like that happens.
Can you see a doctor today, just to be sure it isn't an allergic reaction?
If it isn't, maybe you don't have to give up, could you go back to half a tablet for a bit and then try going up to 3/4?
This is going by the experiences I have been reading about on here, I am certainly no expert, I came here looking for help and advice myself about side effects.
I hope you feel better soon.
Just a thought but usually when I get a cold the sore throat comes first, then the runny nose a day or so later.