Recently, the facebook page changed on my computer (on others from the same router, I get the regular page). I checked the ssl certificate, and it seems to be authentic, however, as the page does not look professionally designed, I am worried it is a man in the middle attack. How to be sure if it is legitimate or not ?
Unless you have installed any certificates in your operating system or browser trust store, you can be fairly confident that any page using HTTPS is not the result of a MITM attack. In your particular case, it sounds more like a CSS resource failed to load, resulting in the corrupted layout you briefly observed.
An attacker who is sophisticated enough to perform a MITM attack against a TLS connection without getting you to trust their certificate manually is likely going to also be sophisticated enough to correctly copy a web page design so that it is completely indistinguishable from the original, legitimate layout.
You should verify that:
You have not installed any certificates that you don't trust.
The domain you are visiting to really is the domain you want to visit.
Your browser and operating system are up-to-date.