This is a very common problem and there is probably a good solution to it! The afternoon sleepiness after lunch occurs generally because the blood sugar rise with lunch causes insulin levels to spike in order to help the body deal with the sugar load. One of the primary consequences of the insulin spike is that it causes drowsiness.
In order to avoid this, the main thing is to change what you are eating for lunch. In particularly, you should try to stick with fruits and vegetables and proteins and avoid large amounts of carbohydrates (rice, bread, pasta) and sugary snacks and drinks (soda for example). The sugars and carbohydrates are rapidly converted to sugar in the blood and therefore will lead to a higher insulin spike and more drowsiness, whereas fruits, vegetables, and proteins will not typically cause this problem.
Another thing that is quite helpful is to engage in some light exercise right after lunch and before returning to work, such as a 15-20 minute walk outside. The exercise also helps to counteract the body's natural 'sedentary' response to the lunchtime meal and will help keep you more alert in the afternoons.
Talk to your doctor
if you have any more questions about this problem!