This is known as a "paradoxical" reaction: https://en.wikipedia.org/wiki/Paradoxical_reaction

From what I understand (I'm not a scientist), caffeine doesn't "give" you energy, but rather makes your body ignore the tiredness, by turning off some of the machinery that would make you feel tired.

By having coffee in the morning and then in the afternoon, you build a "high" of caffeine that forces most of your "alert systems" to shut down. Once the peak wears off though, your body starts remembering to alert you to tiredness, which by then is often multiplied by the fact that by ignoring tiredness you've pushed your system hard when it was already tired. Hence you might feel like crashing.

On top of that, caffeine de-hydrates, so if you've not had enough water, your body will start to crawl.

On top of that, a lot (most?) people will have sugar with their coffee, and often quite a lot of it. Sugar can cause similar "crashes", particularly in the obese (like me), and obviously this combines with the abovementioned items.

Yeah, I have had this feeling as well. I don't think it is a direct cause of the coffee consumption and is more of an association that the tiredness is still building rapidly at the time you decide you need an afternoon pick-me-up. There is a hysteresis in the caffeine actually kicking in leading to the perception that drinking that cup of coffee in the afternoon is making you more tired.

My understanding (I don't have any experience in any bioscience, just an avid coffee consumer) is caffeine works because it blocks receptors of adenosine which has a depressive effect and is a signaler that makes you tired. While the caffeine is blocking these receptors, the adenosine is still building up. So once the caffeine starts to wear off all of the adenosine comes rushing in to those receptors and you start to crash. Drinking more caffeine after you have started to feel tired now takes longer for the caffeine molecules to start to muscle in and start filling those receptors again. If you can start to clear out the tiredness receptors ahead of the caffeine hitting them it will make the caffeine more effective. This has led some people to start taking a "coffee nap" where you drink you afternoon coffee, immediately take a short nap to help the caffeine be even more effective when you wake up.

Are you having coffee with milk? Casein/whey allergies affect a single-digit number of the population and can lead to feelings of sluggishness. Try black/almond/oat/etc.

Have you tried a "coffee nap"? I find that if I just drink caffeine to counteract building tiredness, it's not going to help. Like sponging up water from a broken faucet. But if I lay down and rest (this doesn't have to include sleep, I set an alarm and let my body do what it needs, but the primary factor is giving your brain the peace it needs to process and flush stress accumulated through the day, in pseudoscientific terms), and then drink coffee, I'm reset and it can take effect.

The "Coffee Nap" is where your drink a cup of coffee (quickly, I guess), then lay down for 10-20 minutes to rest, and by the time you get up the caffeine will begin to kick in. I find that process to be a bit more stress-inducing.

Im the same, caffeine after ~1pm results in afternoon sleepies. I have ADHD and caffeine affects me differently.