Re: Lactose Intolerance??
100g (.2 pounds) per week is concerning, for sure. Here is a page of suggestions for the slow-gaining baby. Some suggestions are to make sure that you are letting your baby completely finish on one side before offering the other, to check for food allergies (some things can come through in your breastmilk like cow's milk protein, peanut proteins, soy) and though I assume you're not feeding her solids yet, I'll mention anyway that solids will get in the way of proper weight gain and good health.
Diarrhea would be extremely watery and very foul smelling. There would probably be mucous in it and she would probably be stooling much more frequently than her normal pattern. Here is a page giving some info on what normal stool looks like and what the characteristics of diarrhea are.
Lactose intolerance is so unbelieveably rare in infants. Also, most forms of it manifest within days of birth, so at 3.5 months, you'd be in the clear. There is one form of lactose intolerance that can show up at any time, though. It is called Secondary Lactose Intolerance. According to Joy Anderson, an IBCLC:
So even this other type of intolerance is very rare in bf'ed babies.
[It] can appear at any age and occurs when the intestinal brush border is damaged by an infectious, allergic or inflammatory process, thus reducing lactase activity. Causes of secondary lactose intolerance include gastroenteritis, food intolerance or allergy, celiac disease (gluten intolerance), and bowel surgery. Quoted from Kellymom.com
Here is a well-explained detail on lactose intolerance and babies.
Hope that helps some! I hope you can get to the bottom of the slow gain! :hug