I am sure you are fine and this is just CIC being over cautious.
I got a phone call from the Immigration Doctor informing me last July that I had to go and see a TB specialist as CIC Health had seen "something" on my xray and believed I had TB. I waited 6 weeks for an appointment, went through 2 TB tests and a further specialist appointment until they FINALLY gave me a second xray after 6 months.
The second xray showed absolutely nothing! The specialist signed off immediately telling me she had had to go through the proceedure, but already knew there was nothing wrong with me from seeing me the first time. I received my Health clearance in January.
Be very glad they are sending you for a second xray now instead of sending you to see specialists (which is horrendously expensive).
Please don't stress out and worry too much!