I am an intern returning to a company after having interned here last summer. As such, I have made friends with a lot of my coworkers here, and am regularly invited to lunch with them. I will go with them about a quarter of the time, but I much rather enjoy eating alone and going for walks in a nearby park. I am an absolute introvert at heart, and my lunchtime walks are literally the only time I can see sunlight during the week.
My coworkers and I aren't necessarily close but we are on very friendly terms and enjoy having laughs together, and I've explained this to them, and they say they understand; however, I can't shake the feeling that they feel wronged by my refusals. Am I being selfish?