Is my Mammillaria backebergiana in a bad shape?

It's been a little more than a year since I got this cactus that I have Identified as a Mammillaria backebergiana.

approximately two weeks ago I changed the place of it's pot in my room and took it on a little table but it received less sunlight than it used to. 

A few days ago, I noticed a white spot on a side of the cactus and the top has started leaning on one side too.

I know the white spot can't be due to mealybug because I've tried scraping it and squishing the white texture and no red substance came out.

Do you think it might have something to do with lacking sunlight / could you explain what is happening to my cactus?

Definitely cacti kept indoors need to get as much sunlight as possible. On a south facing window sill is preferred. Your plants are suffering from lack of light, etiolation. This is where the growing tip produces abnormally thin stems from lack of sunshine. If any white spots have woolly looking patches, this could still be mealy bug and a sign that the plants are stressed, and needs to be treated. Investigate the white areas with a needle/ pin in between the spines and probe gently. Get your plants back into decent light/sunshine soonest and they should start to grow more normal, eventually.

Both the main head and the smaller one appear to have been attacked at some stage, with the resulting scar healing white. I don't know if the damage was due to a pest or a blow of some kind, damaging the growing point. In your second photo, there does appear to be something white, not scarring, on a few tubercles.
If further investigation doesn't show any living pests, then grow on and keep it in light and warmth. I can't give you watering advice as I don't know what country you are in, but from the rather shrivelled looking little offset is anything to go by, when it is your growing season, do give it more water, which should drain through quickly.