Don't know about any side effects from lack of lactose, but check with your pediatrician to be sure.
I actually called my DD's doc today in tears because she was shrieking again with her BM today. I was so upset and in tears myself when I called them. I actually spoke with the doc and outlined everything that was going on (difficulty/fussiness leading up to the BM, extremely painful crying during "the push", the consistency, the multiple BMs at once, etc.) We are dropping the mylicon and moving to prune juice in the morning bottle, starting small and increasing if needed. This is to ensure that the BMs are soft - no more putty consistency. Once they are soft all the time, he thinks DD may settle down a little bit.
He did say that it's not uncommon to have babies who just don't like going poop. They don't like the sensation or something and they will scream and cry. His primary concern was to make sure that the stools are soft and easy to pass and to see if this eases her crying.
I'm going to try the prune juice and see if we can soften the stools every time, as when they are very soft, she seems to be able to pass them without crying. We have an appt in 2 weeks, so I'm going to try to find out more information if it hasn't improved. I still am concerned about the crying, but I think I'll do a separate post to see if the BW community can validate his comments.
Thought I'd share as we seem to be have the same issue. Check with your doc. They may recommend a similar plan. Let me know if they say the same thing about babies crying because of the strange sensation/not wanting to go.