I went through a very similar experience two years ago. The night before my birthday I felt a lump where a lump shouldn't be, and I instantly assumed it was cancer. The next day (my birthday) I was the most depressed I've been in a long long time, I didn't have the energy to do anything and the entire day I was worried about my health. It ended up being nothing but this one experience then spiraled and I had health anxiety for months. Every waking moment I was worrying about my health and experiencing some kind of symptom. One of the many symptoms I experienced was the twitching you mentioned. I had this a lot. I too thought that I had MS or some other neurological disorder. Pretty much every symptom you can think of I experienced at some point throughout those months, yet nothing at all was wrong with me.
One tip I can give is to try your hardest to not google symptoms. If you think you have an issue, go to the GP and get their opinion on it, but don't google your symptoms because all that leads to is worry and self-diagnoses. Googling my symptoms is one of the worst things you can do in my opinion.
Another tip I can give is to take notice of the moments when you are preoccupied with something else and aren't focusing on symptoms. It's hard to explain, but let's say for example that I'm experiencing the symptom of a sore throat, I notice that I only actually experience that symptom when I'm thinking about it, but in those moments where I'm preoccupied with something else and not thinking about it, I'm not experiencing that symptom, therefore I must be imagining it.
Also you mentioned that you always have the runs or constipation. This sounds like IBS (Irritable Bowel Syndrome), I have it too. It causes you to either be constipated or have diarrhea, to be bloated, to have abdominal pain, and a lot of other stuff. I'd google it if I were you to see if it matches what you're experiencing. It is very common in people who have Anxiety or Depression, and isn't harmful.
I hope any of this helped.
All the best