What do I do in this situation?

I was on a student health insurance plan that’s payed by the school for 6 months at a time. Typically the policy is in place for that 6 months regardless of whether the student graduates/leaves or anything. I’m a grad student, and my advisor moved to a different university across town. So I then began employment at the university across town (let’s say Uni B) and got on their employer-based plan. Ok, so this new plan was set to begin coverage May 1st. I had a doctors appointment on April 27th where she prescribed a refill of a medication I’ve been on long term. Went to the pharmacy right after the appointment to pick it up. Pharmacy doesn’t have it in stock and has to order it. It arrives May 3rd. Before going to pick it up, I call my new insurance (from Uni B) to get my insurance card information since it hadn’t come in the mail yet. And while doing this the representative tells me “your old plan is still active that’s odd, they never cancelled it”. Both these plans are underwritten by the same company BCBS. So that’s how she saw it. So she told me my insurance hadn’t been cancelled and that didn’t surprise me because typically the student insurance continues through July 31st no matter what because the university has already paid the premiums.

So I decided to fill my prescription with my previous plan that was supposed to still be active (because that’s what the pharmacy had on file and I’d already met my deductible and knew what the copay would be etc). But I find out like 5 days later that they finally canceled my plan and backdated the cancellation to April 30th. Meaning I filled that prescription with expired insurance info.

So fast forward like 2 months. I receive a collections notice from my old insurance company for the cost of the medication.

And now I’m trying to figure out how I can fix this because I was covered by the new insurance policy (from uni B) on the date that the rx was filled. I just didn’t have the right plan info on file with the pharmacy. I asked the pharmacy if they would refile the claim to the right insurance. They said no, it’s too late. So they gave me a pharmacy receipt so I could submit a claim myself. But the pharmacy receipt only shows the copay amount of $30, not the cost of the medication which was like $380.

So I’m not sure what to submit to the new insurance because they want a pharmacy receipt. But the pharmacy is not billing me, my previous insurer is. I’ve tried calling both insurance companies and no one knows what to do, they’ve never seen this.

I should have never trusted that the old insurance was still active, and not picked up the rx.

Anyone know how I can fix this mess?