Is a woman in nidah allowed to go daven at Shul?
Yes, a woman is permitted to go to shul even during her niddah period.
However, it is the common custom for a woman not to look at the letters of a sefer torah (during hagbahah) during her niddah period. This refers only to the time when she is experiencing actual bleeding, and not after the bleeding has stopped.
Sources: See Shut Binyamin Ze’ev 153; Magen Avraham 88:2; Mishnah Berurah 88:6-7. See also: Agur, Niddah 1388; Taz, Orach Chaim 88:2, citing from Maharshal.