Hives

My TB/Dutch WB mare has several bumps on her back in the saddle/saddle pad region. The bumps are about 1cm - 2cm wide ,and maybe 1/2 cm high. I think they are hives, but I am not sure what could have caused them, and what I can do about it? What are hives exactly?

I just bought her 2 weeks ago, so could a new environment have affected her??

Thanks!

Typically hives are an allergic reaction. I sometimes break out in hives when I dealing with hay too much. A few weeks ago my TB broke out in hives, we are unsure of exactly why but the BO(barn owner) had gotten a new load of different hay so she may have reacted to that. One case of hives isn't a big deal but if it becomes a regular thing then I would get the vet involved as the more often they occur the more severe they usually are and if they get severe they can cause the trachea to close making breathing difficult.