Prayers while breastfeeding

We are not aware of any sources that deal with this issue directly. 

While many halachic sources clearly restrict a man's prayers if a woman's breast is in view, there is no similar restriction on other women or on the breastfeeding woman herself.  (With respect to men, Mishnah Berurah 75:3 prohibits; Ben Ish Chai Bo 10 permits in extenuating circumstances.)  Halachot pertaining to a woman's attire for prayer with respect to herself, men, and other women, can be found translated into English in Rabbi G. Ellenson's "The Modest Way", Chapter 4.

Another issue might be the distraction nursing might cause to a woman attempting to nurse and pray simultaneously.  Accordingly, it might be preferable to recite Shema and Shmoneh Esrei when not nursing because of the level of concentration that these tefilot require.  In this context, it could be helpful to review the halachot of tefilah for women who are short on time.  This article provides a handy summary, especially section 13, "Conclusions": http://www.nishmat.net/torah/view.asp?id=46.