Dog sick or what might be happening?

Our beloved pomeranian has always been on a regular schedule. He eats breakfast and supper at the same time each day and has always went out to poop and pee on a regular schedule, always after he eats breakfast and then after he eats supper. Never messes in the house. 2 days ago he did not want to eat his supper but after coaxing he did. That evening he went out really late and pooped. Then about an hour later he went out and pooped again. We then went to bed to find shockingly the next morning that he had diaherra all over the floor. Cleaned it up, he ate breakfast but then did not poop again till last night hours after supper. Then again went out and pooped again late. This morning woke up to same thing of him having diaherra again all over the floor. Now this morning he does not want to eat breakfast. His other behavior is normal and seems like he sleeps the same as before and does not act sick. Have not changed his food or anything and is eating out of the same bag of food that he has had for the past month. Don't believe it is the food because of that. Did notice yesterday that all of a sudden he is licking, not eating some of the weeds? gr that is sprouting up in his yard. Could any of these be upsetting his stomach and if so, how do we safely get rid of them without harming him with chemicals? Your help is appreciated since he is so adorable and loving and sure don't want anything to happen to him. He will be 5 years old this year. Thank you for your help with this.