Sounds like one of the thistles. There are a bunch of different species. Pictures always help with specific ID.
There are others that fit the description, like some of the wild lettuces.
hortster

Here is a link that might be useful: {{gwi:1349240}}

You have what they call stinging nettle. The little thornlike things irritate your skin and it has a stinging itch for a while after. It grows fast. One day it's a seedling and the next it's a foot tall.
Wear long sleeves and gloves to pull it and dispose of it. I never compost it. Get it before it goes to seed or you'll have even more next year.

I have never seen stinging nettle with light purple flowers. Stinging nettle arround here have two different types of flowers one is a long thread that unrolls under the leaves. The thread can be a foot or more in length but is generally only 2-3 inches longer than the largest leaf. The other type has a bush of light colored flowers that appear from the leaf joint. Neither one has thorns on the stems but a fine down that causes the irritation. If it was nettles you can eat them as greens.

I believe Hortster is correct that it is a type of thistle. Many will send up the flower stalk almost over night.

Maybe cut leaved teasel?

http://dnr.state.il.us/education/ExoticSpecies/Cut-leavedteasel.htm

My favorite weed...