hi there
My son is allergic to "something" they think dustmites and we used to use Benadryl, which can cause one to feel not only drowsy but VERY irritable. Benadryl (having taken it myself) makes you feel all hungover and lousy.. which could be how your daughter is feeling (lousy after taking it)
We then switched to Zyrtec, which is much much better. Apart from that, consider this.. The chlorine in your water.. it can be very drying and cause the skin to really get rather dry.. especially when it's too warm (or not) you may want to try more of a sponge bath as oppose to a total dip in the tub (At least for now) see if that helps. I do agree that harsh detergents can cause irritation on the skin, but I definitely think that chlorine makes matters even worse.
Additionally, when it comes to food allergies, people don't necessarily break out on the spot, in fact they can eat something one day and not have an allergic reaction until days later.. I have found that adding more water to a child's diet is most helpful as it allows the body's system to more properly regulate.
Lastly, consider that your daughter might have a little bit too much of candida albicans in her system (in other words yeast) it's not just for we ladies, in fact , men can have problems from it as well... The over-growth of yeast can cause such things as diaper rash (that doesn't readily go away) irritability and much much more... What causes excess yeast? for some, it's different.. but in some cases, too much sugar (in a baby's case) perhaps too much "milk sugar" in addition, so many other foods can cause a reaction such as peanut butter (there is hidden mold) in it that can spur yeast along.. and so many other foods. this includes childrens' medications as they are often laced with sugar of some sort to help the medicine go down (so to speak) lastly... if a person already has an abundance of yeast in their diet, even things like high glycemic fruits (banana) can advance the yeast. basically, anything nice and sweet, pickled, white flour and the list goes on.. Oh a doctor might say, this is nonsense at such a young age, but I assure you, IF you do enough reading on the net, you'll find there is much to be said about the topic.
I wish you and your little one the best!