My puppy is eating raspberries. Could this be why he poops red mucus with his poop?

I recently noticed that sometimes my puppy has bright red mucus that comes out with his poop. His poop will come out with red mucus for a few days and then his poop will be normal for the next few days. After the poop dries, the red mucus is gone. His poop also comes out soft sometimes and hard other times. I recently noticed that he's been eating raspberries that grow in our neighbor's yard, but drop on our side. I haven't taken him to the vet for this because he acts normal. He eats and drinks regularly and has no other visible problems. I took him to the vet a month ago and they "supposedly" inspected his feces before the who pooping red mucus/ eating raspberries thing. This is a whole other issue because after my first visit with the vet he told me to drop of my puppy's feces before my next appointment the following week, which I did. When I went to my second visit, the doctor asked me again if I had dropped it off and I said yes. He then told me the techs probably checked it and would've called me if there were any issues. He's suppose to be a credible vet. Should I trust him or ask them to check his feces again?