When you breed rodents you don't remove them after a few days to re-introduce them gain, you leave them together and you remove one or the other (depending on how your rotation works) when the female is visibly pregnant.
Did you buy adult? Did you buy young rats?
If you bought adult the female might be past due her prime and her nipple showing might be the results of past pregnancies.
If you bought young rats you need to make sure to raise the female to an optimum weight of 250 grams before breeding.
When starting a colony you should buy rats that are 4 to 6 weeks of age to ensure that you have young animals that never bred buying adults might not yield the results you are wishing for.
As for the female being pregnant or not since I don't see the rat I can't tell, females will start getting bigger and it is very visible when holding the female by her tail and looking at her belly (make sure you hold as close as possible to the base of the tail)
Few things to keep in mind
Frequency of estrus- Every 4-5 days
Length of estrus- Usually 12 hours (most often at night)
Gestation 21 days
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