When getting fear or anxiety, why do some people often shake?

Its a symptom that anxiety and fear causes, its also something that happens when someone gets "triggered". Not triggered as in angry triggered as in something sets them off to either stem, or tic, or self harm or whatever it just depends on the disorder. Adrenaline causes it to course through your body. Moving can help cause the shaking to decrease or stop it, but everyone who gets it has their own way to stop it if it happens to the point they have a way to end it.

which ever one is showing doing the same exercise in time intervals such as 30 seconds or a minute several times.

if your boyfriend is cheating on you, my best advice to you is to dump him, he's only going to drag you down. and if you do, don't fret there's plenty better that know how to treat a person right. i mean honestly, the whole point of a romantic relationship is love, and if you love someone you aren't going to cheat on them. i do believe in second chances, but why would he cheat in the first place? i personally feel that if someone cheats, they ruin any chance they wanted/had with you. i wish you the best of luck with this, hang in there.