All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
It was obligatory on your wife to bathe immediately after observing the purity from the two drops that she noticed after ‘Asr prayer and should not wait till the night as she did. Whenever a woman observes purity she has to take a bath and perform prays. Therefore, she has to make up the prayers she has missed before having the bath from her menses. You are not allowed to have sex with her until she takes a bath as Allaah Says: (what means) {And do not approach them until they are pure. And when they have purified themselves, then come to them from where Allah has ordained for you.}[Quran 2: 222]
As for the fact that your wife’s menses stopped, then it is true, but when it came back she is considered as menstruating, therefore you are not allowed to have sex with her till she becomes pure (performs Ghusl).
Allaah Knows best.