I've been diagnosed with Costochondritis. However I'm told that there is a possibility that I could also have fibromyalgia. Is this true?

I can't contradict your GP but I'd be surprised if you had fibromyalgia if your ribcage is your only site of pain.

I've found the following informative website where the symptoms are described quite nicely: http://www.cfids-cab.org/cfs-inform/Ic/costomm.htm

The author recommends some natural remedies at the bottom.

I'd look at Bromelain, ginger and perhaps the glucosamine/Chondroitin first and perhaps consider applying Atrogel^® 2-4 times daily.

Alternating ice packs (bag of peas) and heat packs (hot water bottle) might be useful in stimulating the circulation in the area too but ignore the herbal advice, it's less accurate.
I hope this helps.

Best wishes

Earle