Some dogs are very allergic to fleas - but you will find them on the dog. Dogs scratch at a specific spot if it is a bite - so see if you can find red, swollen or inflamed spots - indicating some kind of insect bite. (Ants, spiders and chiggers come to mind.) You can treat a bite topically with various sprays you can find at the pet store. (And treat for the insects, too!)
If your little one is just itching and their skin is pink - then another issue is probably at hand. If he or she has an odd odor about them, my guess would be a yeast infection. This needs to be treated with meds prescribed by a veterinarian. DO NOT give your dog Benedryl because it only weighs 4 lbs. and you'll overdose it. Skin irritations can also be caused by several other things that must be tested, diagnosed and treated by a veterinarian. Don't wait on this. The problem will not get better on its own - just worse. A tiny dog has the best chance of recovery the quicker you take it to your vet. Hope that helps.
Bathe in original blue Dawn dishwashing soap
We use a flea spot that gets rid of all stages of fleas. We then used this spray and it works great https://amzn.to/2N6lGM2
bathe dog and rinse with vinegar. Be careful because fleas will jump off dog. I always have dog in tub