Can lymph nodes stay enlarged due to past infection such as mono after the infection has passed? i have feelable lymph nodes which haven't changed and there pea sized. i'm also very slim and my nodes are movabl in my neck, groin and jaw?

Yes. If you keep checking your lymph nodes, you continue to irritate them and they stay mildly enlarged. Try to leave them alone. Also, I appreciate that you mentioned that you are thin, which makes it much easier to feel even normal sized lymph nodes. If you develop pain or a sudden change in the size of your nodes, please seek medical evaluation.