Will my cat's sinus infection go away on its own?

My cat suddenly has a very runny nose and is sneezing and has watery eyes. It came out of nowhere! I live in Minnesota and the weather has been crazy lately and spring is finally here, I was wondering if maybe it is allergies. If not, I'm wondering if it is a sinus infection, is it very unpleasant for a cat? I don't have much money and I'm already paying for ringworm medications for him. If it is bearable and I don't have to take him in if it isn't necessary. I would like opinions of cat owners, most of what I could find online were written by vets so of course they say the cat needs to be brought in and given dozens of tests.