At such occassions, do record his temperature and BP. There is quite a possibility tgat he is taking inappropriately low or high fose of anti hypertensives.
Additionally, sincw you mentioned that he is a heart patient, there is a slight possibility that this weakness/body pain is due to statin tablets.
To rule that out, ask him to het a
liver function test done. Also get a complete blood profile done if hasn't been done recently. If done kindly share that.