Would deep vein thrombosis cause pain when straightening leg out?

Maybe. Doctors often examine for increased pain with certain movements to help determine if a deep vein thrombosis is present; however these tests are not very accurate.
See a doctor. If you think you may have deep vein thrombosis you should see a doctor and have an ultrasound test to determine if it is there is one present. Untreated deep vein thrombosis can lead to serious medical conditions and even death.