A lot of sausages contain monosodium glutamate (MSG) as a flavor enhancer. I see it in homemade sausage recipes too. Accent is one brand of MSG that is very common in the stores here in the US.
Some people believe that MSG is the cause of their severe headaches, so you might want to check labels to see if the foods you are sensitive to contain MSG. Some other ingredients that I have seen listed as sources of monosodium glutamate are Hydrolyzed protein,Sodium caseinate, Yeast extract, Maltodextrin, Autolyzed yeast, Textured protein, Calcium caseinate, Yeast food, Hydrolyzed oat flour, and Spice Extractives. I can't vouch for any of that, mind you. I just googled it.
Glutamate is found naturally in protein-containing foods such as meat, poultry, seafood, vegetables and milk. It's said to be partly responsible for the umami
taste, one of the five basic tastes.
Modern scientific studies that I found on the net (yeah, I know this is not reliable) do not find any link between MSG and headaches, but you never know.