With the buds starting to peak out, and the weather warming, your mare's sudden mood change may signal the start of breeding season.
Each mare is different, so keep in mind that the general information offered here may not apply to your mare.
Mares cycle on average every 18-23 days during breeding season, but like humans, vary wildly in terms of length and duration of their cycles. The natural signal mares get that the season has begun is the length of daylight in a 24 hour period, not the temperature.
Behavior changes are common during a mare's heat cycle: she may be grouchy, back-sore, overly interested in other horses, particularly your males (geldings included). She will urinate frequently and hold her tail high in the air.
If you plan to breed your mare, start keeping a journal of her heat cycles as soon as you can. Write down as much detail as possible-her behavior, her physical signs. This kind of record keeping can also help you determine if training or riding problems are related to hormone spikes.
This will give you, and your veterinarian, a good guess at when your mare is ovulating and could save you quite a bit of time and money.
If you don't plan to breed your mare now, but find her particularly difficult during her "time of the month," consider talking to a veterinarian about possible medical or herbal remedies to make your mare more even-tempered.