without seeing the spots could be anything, i presume she doe'nt have them anywhere else? i know its pretty obvious question but it could be nappy rash, you did'nt say if she's toilet trained or still in nappies so its a difficult one, could you give more info?
Could it be a heat rash? it's pretty warm outside, I let my 16 month old be without nappies as much as possible so she can have fresh air on her little bottom.
My daughter who is 7 still gets spots on her bottom and somtimes they are so red and sore she cries.I use a nonbio washing powder and fairy softner and this seems to help, most of the time.When they are at their worst iIuse E45 or an antiseptic cream which works over night.I am puzzled by these and I have taken her to the doctors and have been told to make sure she is dry after the bath and to wear cotton knickers. Hope this helps a little!