My question is -- does she take meds for the allergies? A common side effect of allery meds is sleepiness. Even if she's been taking the same thing for a while, and only now is getting sleepy, the med could be affecting her differently as she grows (people can develop side effects or reactions to meds they've been taking for ages without any previous reactions). So if there is an allergy med in the picture, talk with the doctor about changing or reducing it, or eliminating it if that is possible.
Also think about her level of activity; write it down if you need to and consider whether she is getting more activity than a few months back. Is she in preschool? Have they been going outside there more lately or playing more active outdoor games since spring came?
This also could be part of a growth spurt, which can bring sleep changes and a need for more rest.
And of course, look at her nighttime sleep schedule and quality of sleep. She might need more sleep at night due to a growth spurt, so think about getting an earlier bedtime, perhaps. Kids' sleep needs do change as they grow.