Hypothetical VPN Security Question

I have been using Private Internet Access for a little over a year, and haven't had a single problem. It is a very nice VPN solution.

I do have a question about being tracked, though.

Say, hypothetically, I do something (or go somewhere) while connected through the VPN, and that destination or site is actively being monitored by the NSA or FBI or some other agency. What are the chances that they could trace or track me back through the VPN to my real IP address?

Is there a difference between them actively monitoring and investigating after the fact?

This has bugged me for quite some time and decided I should just ask.

Thanks!

Unless the NSA/FBI has direct access into your VPN provider's network or the provider gives out information about packet decapsulation, they shouldn't see your real IP.