Spotting is very common in the first trimester of pregnancy. As the pregnancy grows it continues to imbed into the endometrial lining causing small breaks in the endometrium. The brownish spotting means old blood which is probably due to that reason. Also the cervix is very soft and full of blood so any manipulation even straining to have a bm could cause a little spotting. Put a mini pad on and if it gets heavy, with cramping and low back ache call your doctor. I would let him/her know at your next visit regardless.
I am an infertility nurse and I work in a major IVF facility. I am willing to answer questions and give guidance, however, since I am not treating you, only your physician can give you the most accurate information.