Aww sis, arm pain during pregnancy is super common actually, a few of us went through it, usually from the weight gain, posture changes, or sometimes even carpal tunnel which maraming preggies get! That said, since it's been bugging you since this morning and hindi nawala, I'd really suggest checking with your OB just to be sure it's nothing serious, lalo na at 5 months ka na, better safe than sorry talaga, yun ang sabi ng aking doctor din. In the meantime, try resting your arm and maybe use warm compress if it feels tense, pero definitely confirm with your doc, ha? Sending you quick relief vibes!