Character moving slowly without carry weight being exceeded.

I encountered a bug where my character moves slowly, as if i had exceeded the carry weight even though my maximum is not exceeded (or even close). I can still jump and fast travel, but cannot run. I am able to run for a step after jumping. Does somebody know how to solve this? I tried to reload the saved game, but this did not work.

UPDATE: I am not in hardcore mode, i do not have crippled legs and there are no effects active.

On the PC at least, Caps Lock key toggles "Always Run."

Did you accidentally hit it and turn it off?

Do you have a crippled leg? (on the Pip-boy check the Status screen, choosing CND)
And are you in hardcore mode?

A crippled leg will slow you down regardless of carried weight, but does not prevent fast travel like being overburdened would.

If you're not in hardcore mode you can heal limbs by sleeping in an owned, or hired, bed (that is, any bed that grants you the "well rested" bonus) or you need to apply a Stimpack to the relevant limb (or use them generally, but this has little effect on the limb), visit to an NPC doctor, use of a doctor's bag, or take some hydra.

In hardcore mode limb injuries are not fixed by resting; you must visit to an NPC doctor, use of a doctor's bag, or take some hydra.

Are you moving all the way down at "overburdened speed," or are you just somewhat slower than you used to be? Switching from light armor to medium or heavy armor will slow you down. Switching to a two-handed weapon will do the same.

Also, since you're not in Hardcore mode, this isn't the answer for you, but might still be helpful for others: if you're close to your weight limit, you may be slowed down when a strength-reducing status effect kicks in. For example, if you have weight 195/200, and your hunger level goes from none to minor, you will suddenly find your weight is 195/190 because of the STR -1 effect.

Also, the type of armor you are wearing affects how fast your 'run' is. If you've got heavy non-powered armor on your 'run' speed can be dramatically reduced.