Frustrated

I don't know what to do. Megan has been potty trained for 2 1/2 years now and has now started to wet her pants again ever since school got out. She will usually have an accident here and there when there has been a change in her life but usually only once or twice, but this has been 2 weeks now and I am going crazy and I don't know what to do. I tell her that if she runs out of clean undies then she has to wear diapers, which scares her, since she is a "big girl." I have even put her in a diaper before when we ran out and she cried and cried & took the diaper off once I wasn't looking, but yet it didn't solve the problem. We make sure that she potties before we leave anywhere, and when we see her drop to the floor (which means she has to go) we ask if she needs to go and she will say "no" and this will go on until she actually admits that she needs to go or wets her pants. What am I suppose to do? I am losing my patients with her and I feel like I have been yelling at her a ton lately. Which I know probably doesn't help the situation, but how do you be kind, loving and supportive to a 4 1/2 year old who KNOWS that they need to go to the bathroom but just doesn't want to until it is too late? I am out of ideas, needed to vent and hoping for some advice... have any?