As somebody who recently has also had the most frustrating bowel issues that went of for months, go back to the doctors but ask to see a different GP. Getting a second opinion can do the world of difference. The usual doctor I see also told me "It's probably just IBS" because my blood test came back normal too; but in my opinion, IBS is just a list of symptoms and not a diagnosis of anything.
When I saw a different doctor, he was so proactive in really getting to the route of my problem! So yeah, if you get to see a different GP, really stress to them how much your problems are effecting you and how the previous doctor wasn't much help. Don't let them just palm you off with "It's just IBS" or "It's probably just stress" as it's their job to actually help you.