I suspect, but I am not sure, that my dog may have an urinary tract infection.
I brought a urine sample to my vet this afternoon.
She called me to schedule an X-ray for tomorrow. Although they were slammed, she is squeezing my dog in at 9:15 a.m.
What are the possibilities of what they could suspect? If it was an infection, they would have just prescribed medication; therefore this must be more serious, right?

Not necessarily. They are probably just trying to rule out all possibilities. Just don't work yourself up and put things into your head to make you more nervous.