Co-Pay Changes?

I am on a Medicare Advantage plan and I have a PA for OSA. I have had the absolute worst experience trying to find out what I would pay for the prescription. I have been given so much misinformation by my insurance company. I initially decided to self pay after hearing about the Bridge program but due to a diagnosis of OSA I don’t qualify for the program. I decided to go ahead and use my insurance and see what price they would come up with once it was filled and ready for payment. The price last month was $336 which included my $250 deductible. The additional $86 was my copay. Before paying I called my insurance company twice and they confirmed that once I paid the deductible my monthly copay would be$86. I went ahead and paid. I just ordered my refill and now the co-pay is $175! Does you co-pay fluctuate from month to month? I talked to the person at Express Scripts and he could not explain other than to say what the charges were. I don’t know how a person is supposed to make a decision on taking a medication if the cost changes monthly.