My single friends do not ask me out anymore since i am in a serious relationship?

I feel like since i entered the serious relationship I am in the single friends I use to hang out with don’t seem interested in hanging out with me anymore. Now I feel like I always have to be asking them what they are doing and what they are up to. It hurts.. How do I solve this?
Is it that they are jealous or intimidated by my being in a mature relationship ? What is it?... Can anyone answer this? I am female btw.
It is true that as a result of being in this relationship I don’t have as much time for going out and my relationship is my priority as I am in a part of my life where I want to get married (i.e late 20s).
However when I am available I always try to contact them.. I have also asked them to let me know when they are going out just incase I am available but they don’t unless I ask them…
What could be the cause of this?... Is it that they weren’t really my friends in the first place?