Yeah, it's very hard to get a Crohn's diagnosis without a colonoscopy. Blood/stool tests can test positive for inflammation, but they can't tell you what the source of the inflammation is. What stool test did you have? Was it faecal calprotectin? A high calprotectin result is usually automatic grounds for a colonoscopy.
As far as the pred is concerned, a 30-day course is quite short for pred. It's not a benign med but it's unlikely to have done much damage in 4 weeks, unless you already had a latent infection which pred made worse. I suspect between the herniated disc, active inflammation, and stress you were already pretty run down, which in turn made you more susceptible to catching infections.
Dx Crohn's in June 2000. (Yay )
Tried: 5-ASAs, azathioprine, 6MP, Remicade, methotrexate, Humira, diets.
1st surgery 20/2/13 - subtotal colectomy with end ileostomy.
2nd surgery 10/7/15 - ileorectal anastomosis. Stoma reversed and ileum connected to the rectum.
Current status: Chronic flare. Do I have any other kind?
Current meds: 50mg 6MP; Entyvio (started 3/11/16)