It takes a good six weeks for the levothyroxine to reach an optimal and stable level. That is why your labs at first are usually 8 weeks apart. I have been on it five years and I very rapidly reached the correct dose and felt normal after starting it. With only a few minor dose changes over the years, I feel exactly like I did before I got Graves.
That being said, my irritable bowel and migraines and anxiety and allergies, all of which I had preGraves, have not changed either! Especially after having been on an antithyroid drug like methimazole for awhile and changing to a hormone, I would not be surprised at a flare in your GI and other pre Graves conditions as your body adjusts and things settle down. There may also be a need to adjust your dose and you will be able to determine that at your first labs.
As Graves seems to be triggered by stress and anxiety, especially in people with other autoimmune issues like allergies, etc., it is sometimes hard to know if symptoms are caused by thyroid hormone levels or anxiety, allergies, IBS, etc. In the meantime, while you wait for your first labs, treat those other conditions like you would have treated them preGraves.
It is usually pretty easy to find the correct levothyroxine dose. Wait an hour before eating after taking meds, no supplements like calcium for 4 hours, no biotin, and be aware PPIs like Nexium will affect absorption also. I have no idea if Miralax affects absorption, but would doubt it; you might take it a couple hours post levo just in case if you can.
Hang in there. You should be back to your baseline normal soon.