University Police Say I Accused My Ex Of Having Cp

So basically long story short i got a call from the university my ex attends' police department and he told me that hes in contact with the fbi and protonmail and says i sent emails accusing her of CP. He even told me that protonmail says the IP addresses of two email accounts: thatgirl6@protonmail and thatgirl7@protonmail (i dont remember the exaxt emails) belongs to me and that protonmail validated this. The thing is, i definitely didnt do this.

Here is basically a shortened version if the call:

Police: "im here to talk to you about emails you sent" Me: "what emails?" Police: "emails about miss (her name) having CP" Me: "i definitely didnt send any emails about that" Police: "im in contact with the fbi and protonmail and they said the ip address belongs to you" Me: "thats weird" Police: "look man if its a prank thats okay i just need to know if i need to investigate this as im in charge of child endangerment here" Me: "i mean if she needs a vouch or something id say that i highly doubt that is something she'd do" Police: "ok, then why did you send the emails" Me: "i didn't send the emails"

However, theres a lot im confused about.

1. Why is the university detective calling me?
2. Why would he be calling me and asking if it was a 'prank,' wouldnt investigating her be first priority?
3. Why would the FBI involved? This is in the same state and i dont even know why they would use their resources to find out who sent an email regarding an accusation, unless they deem it harassment which still seems odd.
4. Protonmail is a swiss company. They went through international law to get a court order for this?
5. Does protonmail even store IPs? I know they log if and once swiss court demands it.
6. How would he know the IP is mine? He got a subpoena from my ISP?
7. How did he have my number? Im only 18 and my number would be registered under my parents name. How would he know which is mine without contacting my parents first?

To me it seems like what actually happened is someone accused her of having CP and they already disproved that claim, so then she reported it to the campus police and she suspected that i was involved. That seems the only way they would even consider calling me because it just all seems unreasonable. Also the behavior of the detective seemed like he was fishing for an answer based on what he was saying. Am I crazy for thinking this? Should i be concerned that he wasnt bluffing and my IP was somehow connected to this?