I'd be tempted to ask if you have suffered any other symptoms that might be associated with overtraining, since you exercise almost everyday.
However, I estimate that the 50 lengths add up to around 1250 metres and that takes you around 15-20 minutes, so the volume is not excessive.
Can I ask, though, if you are trying to complete the distance as fast as possible every time? If so, then speed work should only be conducted a maximum of 3 times a week, even in a very fluid (non impact) sport such as swimming.
Please ensure you have variety in your training, so some long, slow sessions as well as the shorter, faster work.
It is possible you'll become overtrained, with the associated failure to recover between sessions, if you are thrashing back and forth every swim and even more likely if you don't balance your fitness regime (a form of stress) with your stress levels at home and at work.
In other words, swim hard (longer or faster) when the rest of your life is good and swim easy (shorter or slower) when life is hectic and stressful.
So, assuming you train in that balanced way, how is your sleep?
It should average a minimum of 7 hours, preferably uninterrupted. If you fail to achieve that, you may not be recovering fully between sessions.
Can I recommend our Dormeasan® if your sleep is poor. I use it following strenuous training on the bike.
It is an unfortunate truth that recovery slows with ageing. I don't know your age but Delayed Onset Muscle Soreness (DOMS), the ache we all experience in the two or three days following a hard effort, doesn't resolve as fast as when in our youth.
I find 5g of an amino acid called L-Glutamine after hard sessions excellent for speeding recovery.
Finally, do you ever take a week easy? If you train consistently, day in day out, week in week out and at the same intensity, you will become good at swimming at one speed over one distance.
It may be a good idea to take an easy week and begin to add some variety after that as outlined above.
I work on a 3 day cycle of Hard, Moderate, Rest and take an easy week every month (I try to time it to coincide with family commitments, work trips away from home etc).
As a result, I haven't been ill this winter and though fatigued and tired from training, haven't tipped over into prolonged muscle soreness.
I just had to make sure the first 90% of this answer was mentioned.