This could be many, many different things, common causes include neck problems, migraines can be brought on by stress and/or trigger foods, dehydration and skipped meals, anxiety about receiving food brought by others could also be a big trigger. High blood pressure can also cause regular headaches, and it can be a side-effect of medications and caused bby alcohol or drugs. Too many painkillers can also cause rebound headaches.
Worries that others are trying to poison you can be caused by some mental health conditions, examples of paranoia include fear of food/drink being poisioned, paranoia can occur on its own, or in schizophrenia spectrum disorders, borderline personality disorder, even depression and anxiety can include some paranoia. That doesn't mean that the headaches are not there - it just means that other people are not deliberately causing them.
Some medications can have paranoia as a side effect. It could be that the headaches have a different cause than poisioning, and that nobody is trying to harm you.
Things can you try to help work out what is happening:
get a basic health checkup - this is the most important thing you can do and probably won't involve expensive things like blood tests but will quickly check the most common causes of headaches
record your recent medication history, including whether any have changes since symptoms began - do not stop drugs, just keep the record update if anything changes, some medication can make you very ill if you suddenly stop it
- avoid alcohol, smoking and drugs that aren't prescribed by a doctor (like cocaine, cannabis, diet pills)
- check the 'patient information' for your medications, and how common each side effect is, psych med details can be found here - written by the people taking the meds - nobody gets every symptom
- get your blood pressure and eyesight checked, quick and cheap - a basic health check will do this
- see if you have pain or stickness around your neck which might be causing it or contributing to it, see a physiotherapist or try yoga etc
- add monitoring your stress levels to your food/drink diary
- check your food/drink diary against common trigger foods for headaches, including snacks like chocolate and drinks like coffee
- ask if anyone else in your house gets regular headaches, or if any blood relatives do, and if they know the reason
- talk to your relatives about your worries that the food or drink is triggering your headaches, and how to fix it
I really, really think you should get a basic medical health check as soon as you can. Things like blood pressure only take 5 minutes to check and many people with headaches, and many people with mental health problems in the past don't get regular health checks.