Hello Josh Sophie,
Thanks for posting on HCM,
From your description it is possible that it is a cyst
that has developed. But there is also the possibility that it could be a developing abscess
(due to infection). You will need to visit a surgeon who can examine it physically and decide if he wants to excise it or biopsy
it for analysis. It could still be a malignant mass.
If a developing abscess or myositis
then a course of antibiotics should be indicated to treat and probably get the abscess incised and drain with it collects into pus.
Hope this helps