PA workman's comp issue

I am an RN in an office. 2 weeks ago; I stood up from my desk with a few "light" charts in my hand & my knee popped & I couldn't walk or bear weight. Needless to say.... I tore my meniscus. I was told to report this to comp because it was a workmans comp issue. Workman's comp filed a claim # & have been in touch. I simply stood up from my desk to intake a patient & pop. I need to have surgery in 2 weeks & our Comp. Carrier notified me today that they are investigating & potentially rejecting the claim because they do not feel it was work related. I have had NO injury to this or complaints prior. It happened at work & I was seen by the physician that day & 3 days after. I have continued to go to work limping but the ortho said I need the arthroscopy soon. I have been with my employer for 9 years now. I am very concerned because I do not want to be stuck with a huge bill that I cannot afford. I cannot imagine that getting injured while at work & also having private health insurance should cause these worries about who will pay! . I don't know why this is happening or if they can reject this. And what are my options? I appreciate your time." Jillia