This sounds like something I too have been troubled with over the years. Swimmer's elbow (epicondylitis) usually due to one of the following (or a combination of all four):
1. A sudden increase in distance.
2. The addition of large paddles or a large drag suit without a gradual build-up.
3. Missing a week’s training and jumping straight back into
your pre-lay-off distances.
4. Technique issues.
As a regular swimmer you have an expectation of what you want from each session. Fitness may be high and 3,000m may not physically take its toll straight after a session. Undoubtedly, though, there may be ramifications if a gradual build-up isn’t taken as you return to pre-lay-off distances.
If you’ve been swimming regularly, consider the arm cycles the shoulder and elbows have been through over the years. You’d literally have swum hundreds of thousands of repetitions possibly with less than ideal technique.
I spoke to sports doc and triathlete Ruth Burnett regarding this issue and she recommended the normal RICE (Rest, Ice, Compression, Elevation) procedure. This would be an ideal treatment for a symptom that’s possibly lateral or medial epicondylitis.
However, it’s hard to be specific without a closer examination, so you should see a physio or sports doctor.
During the rest phase, try to keep your swimming going but at lower intensities. Having a swim coach check your swim technique may also be very useful. The first part of the pull phase is considered to be from slow to fast, so a sudden and aggressive early pull phase will place the elbow under some stress as you set up your catch. Swimming with an early vertical forearm – if set up too strongly – may irritate the joint and tendon.