It could be your insulin to carb ratio is wrong at lunch time, or your carb count isn't accurate. Maybe also because it's warmer, your insulin is more effective. I know the heat affects some people. Have the lows started recently or have they been happening for a while?
How are your levels for the rest of the day? Has anything changed, eg are you exercising more before lunch, lost weight etc?
If your lunch contains a lot of fat, that will slow the rate at which food raises your blood sugar.
If you haven't done so already, I'd ask your hospital team for advice?