What Causes Severe Migraine On Taking Atenolol?

I was taking 120 mg propranolol in 40 mg doses 3x/day. I asked to change to atenolol b/c it was on the $4/$10 list at the drug store. My doctor changed me and I picked up the scrip. It says to take 1 - 25 mg dose per day. The first day was yesterday and I woke with a very severe migraine this morning. I took my 25 mg dose again along with the rest of my usual meds and added a tramadol. Pain is much better now. Is the 25 mg dose of atenolol a comparable dose to the 120 per day dose of propranolol?