Exercise and Insulin

I'm type 1. I recently switched to Novorapid and Lantus Solostar. But when ever I walk or exercise I'm not able to do more than 15 minutes as my blood sugar goes really low sometimes even when I take low units of Novorapid and sometimes even when I skip it. Any idea how to manage my blood sugar as it goes as low as 20, 30 and 40. Sometimes I'm not even in my senses to do things on my own. My doctor asked me to eat slow sugar releasing food like oats and brown bread but it doesn't seem to be helping. Any suggestions??

No idea about basal testing. I usually take Lantus Solostar at night and in morning I'm having severe hypo.