Given that the reaction is exactly in the distribution of your mask, I would suggest that you speak either with the company that provides your CPAP machine, or the doctor
that is currently managing your care and the use of the CPAP. It is quite likely that the mask that you are using is causing the irritation. The company that provides the machine can either adjust your mask as needed, or provide a new face piece that might be able to alleviate your symptoms.
It is possible that you do have a minor allergy to the plastic that is used, but unless you are having other allergic symptoms as well, a visit to an allergist would probably not be absolutely necessary given that there are other, simpler, things to try first. Using your CPAP machine is an excellent thing to do given a history of obstructive sleep apnea, as research continues to demonstrate the many risks of uncontrolled sleep apnea leading to cardiac and many other medical complications. There are also surgical options that are being perfected, and speaking to a head and neck surgeon
(aka, ear-nose-and throat surgeon aka otolaryngologist
) might be a good way to review some permanent treatment possibilities.