In his army days, if bed bug found, soldiers put the mattress/bed out in the sun in the morning and bring back in the evening. Just once. Bed bugs would never come back again. This is heuristics, not science.

For a healthy person (including grandma), the skin reaction should subside gradually. Otherwise, blood test needed to identify other issues.

Sun radiation can aggravate the skin irritation.

Dr Yii pointed out two solutions:
A) ask pest pro to clean up. They should know how.
B) eye doctor or skin doctor

The government agency for toxic control is HSA.

–input from NEA
If you don’t travel a lot, then most likely they come from the neighbors. If building is not very old, then they don’t go through cracks but rather the corridor.

During hot and humid months bed bugs are more active. In recent years it’s May to Oct.