Hi All!
I am having an odd problem and would appreciate an suggestions. I have been walking Bear (my 5 month old yorkie) since he was 3 months old. I live in Chicago and walk him on grassy areas on the lakefront. I used to walk him a mile or two and he seemed to love it. It got rid of some of that endless puppy energy.

I walk him on a 26' Flexi leash so he can run back and forth or chase the mini Kong tennis ball I bring for him. He really plays hard, tumbling, jumping, panting etc..
Everything was going fine until roughly 2 weeks ago. We were walking across a huge soccer field and he started to really freak out. He was frantically biting my ankles (hard!), barking very loud at me and panting very hard (he is always panting on a walk). When I stopped walking he laid between my feet looking up at me just panting. If I started to walk again he would start to do it all again. Really loud bark and really hard biting. Very out of character for him. I thought maybe he hurt his foot or something so I picked him up to carry him home but he kept barking at me so I put him back down on the ground. I waited a few minutes thinking maybe I had walked him too far and he needed to rest. When it looked like he had recuperated I started to walk back home and he seemed alright. I didn't really understand what had happened and attributed it to him just being tired. He was not in any type of pain that I could see. I could not nail down a good reason other than being tired so I decided I would just not walk him that far anymore and went home.
From that day forward he has done it on every walk I take him on. Every time a shorter distance until finally today he did it across the street 1 minute outside the front door. The only thing that is different from the first occurrence is that I eliminated exhaustion as a possible cause because he did the behavior and then ran around in a large circle as fast as he could for about 3 minutes. He was looking like a crazy dog while running too. In the 2 weeks (I walk him every day) that he has been exhibiting this behavior I have been able to eliminate the following possibilities:
Hunger - He is never hungry as he is well fed.
Water - He gets plenty of water as there are water fountains on the paths that I give him water from so he is not thirsty.
Temperature - It has been between 45 and 93 and he does it no matter the temperature. It just so happened that the first week he started doing it temperatures were high.
Sunshine - For a while I really thought it was sunshine as he would run to the shade. It was very hot here for about a week. But he does it on overcast and cold days now as well.
Distance - I thought he might be too far from the house but he does it at all distances.
Location - It is not certain areas or certain routes as I have changed it up since this started.
Harness - I loosened his collar and harness immediately with no change.
Medications - He did get switched over to Trifexis at about that time but I seriously doubt that could cause this. Since he has been switched I have read some about the drug and am thinking of taking him off of it for other reasons but I still doubt it has anything to do with this particular problem.
This is really becoming a big problem as I can't walk him more than a half block now without him freaking out. I really don't know what to do. He seemed to love his walks. Even now he looks so excited when he is going out. It would really be ridiculous for me to go take walks without my dog but I can't have him go crazy every ten feet. I would hate for him to miss out on this nice summer as Chicago has some brutal winters.
Any help or suggestions greatly appreciated!