this happens to me quite often, experiencing really bad anxiety for no reason, and usually it latches onto something i really am anxious about so it just gets worth.

i've found it can really help if you have someone you trust who understands your anxiety issues to talk to, especially if they can reassure you in some way (tell you that you are okay (even if you're not) or that it's okay to be feeling the way your are)

other than that i haven't really found anything that stops the baseless anxiety, but sometimes different coping strategies will help (listen to music, sit in a place you feel comfortable, hold an object you find comforting, watch a movie you love, etc)