Oh, yes, the PM makes sense now. Thanks for volunteering me, bubs!
My experience was quite different. No swelling or heat, for one, and moderate but consistent lameness. The thing about the deep digital flexor is that it is, well, deep. Damage to it, unless I suppose it is extremely severe and affecting more superficial tissues, should not be visible externally. Because of its interior location in the leg, it is also impossible to ultrasound.
In my mare's case, we did two rounds of X-rays and two rounds of ultrasounds, and nothing ever showed up as a problem--she looked clean. The vet thought she might have damaged her sesamoid ligament, so injected that area, but nothing helped. We turned her out for several months and then tried stall resting. When she was still lame after six months, we hauled her to the hospital for an MRI. There they found that she had three tears, at least one quite large, in her deep digital flexor tendon of her left front leg. The biggest was seated under her navicular bursa. Another was way up on her cannon bone.
They gave her a 70% chance of return to soundness if we followed their protocol. Treatment included IRAP injections and two courses of shockwave therapy, in addition to six months of very gradual and slow transition from stall rest to solitary paddock to turnout. Then I began to ride her again.
She did come back that season, but she never felt "right." She actually won a big barrel race. Then out of the blue one day she came up dead lame on the same leg. A return to the vet showed, well, not much of anything again. More stall rest, more shockwave. We've repeated this scenario many times now. "Off" one week, 98% sound the next, here, there, everywhere. To make a long story short, that is. Every time the vet thinks it's something different--but he can't be certain, as again, ultrasounds and radiographs are clean. Her collateral ligament, he says, her suspensory....based on, what, a hunch? No one really knows.
My mare's original injury was in May of '09. She was diagnosed in October of '09. I went back to riding her in March of '10, competed on her throughout the summer, and she was badly re-injured that August. It's been on and off since, now for a year. I've had to retire my nine-year-old talented mare and give up the sport I love. I suppose that was not the happy ending you were looking for.
The ultrasound, in your case, is a smart move, but keep in mind that even if there is a problem it may not show anything. If she is sound now, though, the injury may be well on its way to healing (and thus not show up), or the lameness and swelling may have been from something else entirely.
If she IS sound, and vets clean, since you have no proof that you had a major problem in the first place, you could probably go back to light riding, gradually increasing the workouts. If she becomes "off," listen to her and scale way back, or go back to stall rest and the vet, better yet.
If they DO confirm a soft tissue tear, as about the treatment option shockwave. Also look into therapeutic shoeing to minimize the stress on the injured tissue, and you might discuss some of the new injections, such as IRAP and PRP.