Inner elbow/bicep tendon pain from drop bars? I've just recently bought a new bike with drop bars for my daily commute. It's great fun and actual feels quite comfortable after following a few fitment videos and guides online.

The bike is a single speed Kona Paddy Wagon.

I have no pain or strain while riding, however at night the tendons on the of inside my elbow tighten up and get quite sore when used. It is really hard to pinpoint the pain, but it's not muscle, it's tendon. However the pain happens whenever I bend/flex my arm with the palm down or thumb up. When the palm is up, it doesn't hurt.

I'm not even 100% sure it's because of the bike, but the position my hand is in during the pain is basically the the hood position, so it makes sense.

Had anyone experienced this? It's what I imagine tendonitis might feel like.

During my attempt at setting things up, I managed to take a couple photos, I don't know if they help or not.
Tendonitis (tendonosis) is one possibility, however I would expect some pain during riding especially during the beginning of your ride. When you say biceps tendon I assume you mean near your elbow.

That said, the first things I would investigate are ulnar nerve irritation or compression at the cubital tunnel a/o the ligament of Struther's. The ulnar nerve weaves through several fibrotic "soft" tissues in the upper arm where entrapment can occur including the medial portion the triceps. The vibration of the road (absorbed by hand, triceps, and FCU muscle mentioned below) coupled with wrist extension and wrist radial deviation can put sustained tension on the ulnar nerve which can cause entrapment and damage.

The flexor carpi unlaris muscle can also contribute or cause this problem.  Repetitive inflammation of any tissue in the body results in the development of scar tissue which can occur between muscles, fascia, and within the sheath through which large nerves travel. As scar tissue continues to develop, the restrictions or fibrosis will increase in these areas.

Go see a LMT or chiropractor who is certified in Active Release Techniques so they can help break up the scar tissue and get that nerve moving more freely.  Surgery should be a last resort in this and most other repetitive use musculoskeletal injuries. This is related to and can be exacerbated by cyclist's palsy, and Tunnel of Guyon entrapment of the ulnar nerve. I experience this problem as well, mine is worsened by core weakness and too much pressure on the hands. Higher handlebars will help get some pressure off your FCU and triceps as well. They are ugly as sin, but those long tall quill stems are helpful. 

Ride On.

Eric Poehlein DC