Ladies I need your help

I need your help with some knowledge put me on game.. I have suspected my significant other of potentially being having another person in his life he’s been changing his phone background everyday and changes it before he walks in at the front of our apartment and he has changed his personality meaning he’s been finding more and more to not like about me and isn’t as worried about seeing me when he’s free as he use to I am just not as much of a priority so I guess my question is how I go about this situation