Melissa,
I hate to say this to you, but I honestly think that you need to take a further look into this situation with your little girl! Let's not presume the worst that she's a masto patient. She may not be. My husband do has issues with his skin in that he has urticaria and darier's sign and I'll bet anything that he's got Chronic Urticaria. He get's itchy with the cold and will get hives too. Does he have masto, NO WAY!
Masto is a different thing - it's a genetic disorder and although they do have an autoimmune form, since there's not been any research done in this there's no way to know to what extent this is limited to being purely autoimmune - I'll bet anything it's also a genetic disorder with an autoimmune angle to it. So, this means that there's a small chance your daughter could have masto, if you do, but it's so rarely found in families that the doctors even wonder at this.
But this doesn't mean you should ignore this. Your little girl shouldn't be seen by a pediatrician because they don't know this disease well, she needs to be seen by YOUR doctors. They will know what to do to track this down and to medicate her. Although this may be nothing more than an atopic dermatitis, which is what this could very well be, it's still enough to drive her nuts and she should be medicated for it. I've got several of the children of my school (I'm a school principal) who have atopic dermatitis and I've talked with the parents about this and want them to take their children to see my doctor so that these kids can be put on a medication regimine - there are needing it because they go borderline anaphylactic at times - definite asthma attacks!
So, Melissa, your girl needs some investigating to figure out what is going on with her. She need some medications but as you know, you need some doctors do figure all of this out first! Until then, keep the water at a temperature that is not too hot but also one that won't chill her. I would suggest you use the shower instead of the bath. They are quicker and won't cause her skin to get so involved. Also, when you dry her off, pat her dry, don't rub. If she does have any kind of skin issues, be it a dermatitis or masto or whatever, rubbing will only make things worse.
So, she doesn't like a shower? That's only because she's not accustomed to it. My children have never known bathtubs because the homes we've lived in here in Brazil only had showers and since my babies were infants we had a little infant bathtub and I'd put it on the floor of the shower and bathe them in it. Once all threat of drowning was over I'd leave the tub in the shower and could let them sit there with the shower on, so the shower was never an issue. If your little girl has a hard time with the shower idea, then put a bathing suit on and get in there with her. If she sees that mom's doing okay in the shower, she'll get the same idea quickly and if you leave toys on the floor for her to play with, she won't miss the bathtub one bit, I promise you! You'll get tired of the toys in the shower, but that's a small price to pay if it helps her keep her itchies down!
Lisa