My rottie has bad diarrhea :( help?

I feed my 5 yr old rottie beta adult dry food with pedigree chum, is it possible that the dry food is too rich for her, would that cause diarrhea? about four weeks ago she just started doing...for lack of better words....really big poops, this week i took her to the park and she had explosive diarrhea about three times, it may have been from excitement though on this occasion to add to the problem as she hasn't been to the park in ages. The thing is i love the beta food that she has because she has a really good coat with it, and i don't want to chop and change her diet to make things worse. would beta light adult help the matter? or is it more the ingredients? any help much appreciated, thanks x