From the information that you have given it seems that the two symptoms are different and due to different causes.Two weeks ago you probably had contact dermatitis
to poison ivy,but the present symptoms suggest that you might be having some allergic reaction
to some food products.Sometimes allergic reaction to food products can cause diarrhea
, stomach pain along with hives over the body.You will need to take anti histamine
tablets and apply calamine lotion over the hives.I would suggest you consume bland food for now,consume adequate amount of water to avoid dehydration
and consult your allergist for further course of action.
Hope it helps,
Dr.Soumya Nagaraja MD