You just get used to wearing heels. I worked 2 jobs in 3 inch heels for 20 years. 8 to 5 and then 6 to 2. Flats felt funny. Took me awhile to get used to flats but now that I am heels are hard again.
Flats, for those who wear heels most of the time, feel odd because the calf muscles.tendons are stretched down in flats. When you wear heels alot they are accustomed to not being stretched out.
Also, as you might know, anything other than leather for heels will always create achy feet and leg aches. One young gal at work wore vinyl fashion boots all day,.....she could hardly walk to her car at days end!