Whatever happen to your dog, the first thing to do - is to measure her temperature. Every dog's temperature is slightly different, and it's very important to know which is normal for your dog. Measure when your dog is healthy and relaxed. It is very easy to do, everything you need is a thermometer, a little bit of vaseline and sterilizer (some spirit). Ask your puppy to lie down on her side. Sterilize the worling end of the thermometer, put some vaseline to smooth it and push gently into her anus and hold her down for 2 minutes. Everyone can do it before taking your dog to the vet, the majority of the viral deseases start with the rise of bodily temperature.
Warm weather causes many problems (food poisoning; besides lethal infections carried by ticks, their bite could be a reason for asthmatic allergic reactions; lack of raw meat causes vitamin deficiency; warmth helps infestation of worms, etc), and if your dog is lethagic, it could be difficult to recognize sometimes, whether the reason is the outdoor temperature or it was caused by some illness. If she is hesitant going up or down stairs - something makes difficult the belly muscles to contract. Her belly may hurt - it could be a gut infection. But she could also have swallowed something - you'd need to X-ray her for that.