This is Dr. Christian. Welcome to JustAnswer. It sounds like you are seeing fleas. They are very small and very quick. They bite, suck blood and cause itching. Here's a link about fleas:
Here are some suggestions for flea control. To be effective, 3 important steps must be taken:
1) Controlling fleas outside
2) Controlling fleas inside the house
3) Controlling fleas on your pet
Each of these steps target different steps in the flea life cycle that includes egg, larva, young adult, and adult. Targeting different steps of this life cycle will lead to quicker flea control
Pet Flea Control (All of these products are available through your veterinarian)
Over the Counter Flea Control- A large number of flea control products exist in the "over the counter" market. Most are variations of permethrin or pyrethrin products which have been around for years. Due to resistance developed by fleas to these products they are not nearly as effective as they used to be. Additionally, young and old pets and especially cats can be harmed by these products. If you decide to use these products be sure to follow the label directions exactly and only on the animals and ages they are labeled for.