Suggest Treatment For Bowel Movement Problem In Infant

My 6 month old has been exclusively breastfed until recently, she has ALWAYS struggled with bowel movements and has NEVER been regular. I would stimulate her when she seemed to get really uncomfortable. The introduction of food seemed to actually help, she started to go more on her own more often (daily)
In the last few days her bowel movements have been hard and smelled fishy. She is very irritated and crying non stop. I have been mixing infant probiotic with her food and still breastfeeding her throughout the day and night. What could be causing her discomfort? Her belly is not distended or hard, but she seems so miserable...what can I do?