Hello XXXXXXX

I am sure it just appeared like hives or urticaria and getting responded to benadryl.
Benadryl should be used with caution and no more than 2 times a day as it can make child very sleepy. Itchiness does suggest it as urticaria, but if it is recurrent, there may be a chance of atopic dermatitis. All rashes (except some bleeding related) even fungal , will be itchy.
I am only worried about they being painful. painful rashes isn't a common feature of allergy. Allergic rashes do get painful only when the scratching causes wounds or gets infected.
painful rashes often suggest cellulitis, sometimes like in eryspipelas following a bacterial infection and may need antibiotics. Even herpes can be painful.

If the pain is negligible, the diagnosis rests with urticaria. after clinical confirmation of the rash, a skin specialist will usually also start with medicines like say syrup atarax than benadryl or also some oral steroid as a short course.

If the pain is severe, its important to rule out infection and/or consider need for antibiotics. Infections usually come with fever.

It will be more comfortable to get more precise if you can provide a pic of the rash.

I hope this answers your concern.

Please write back if any doubts.

thanks

---