I wouldn't delay seeing your GP for confirmation. If it is chicken pox, think of the other people in both the airport and the plane, it is a serious illness for pregnant women to be in contact with and can lead to shingles in adults. Being an airborne disease, coughing or sneezing can easily pass it on to others.
Normally travel insurance will pay out for cancellation, less any excesses which will be deducted in respect of each passenger and under each section of the policy claims are made. They wouldn't normally pay out for the difference between the original booking and any re-arrangements made, although you need to read the policy document to confirm that is the case. Furthermore, if you only have Single Trip insurance, I believe that you will probably need to purchase a new policy for the re-arranged holiday should you claim on the old policy.