I have developed a relevant increase in myopia and a blurred vision due to light sensitivity 3 days after cataract surgery. What can be the reasons?

Not uncommon. The eye is in the early healing stages and is usually accompanied by some inflammation. Post op eye drops help. A very red eye and or pain would be reasons for concern. Any severe decrease in vision would be a reason for concern - and reason to see your surgeon.
Post op period . surgery was done 3 days ago it is going to take some time for the wound to heal and the intraocular lens to set in place it will get better in 4-6 wks.