What determines how sweet or sour your period smells isn’t the blood itself but rather the vaginal mucus there at the time. At different points in your cycle, your vaginal mucus changes in consistency (from watery, thin mucus to thicker, whiter mucus) and when this change occurs, the pH of the mucus changes too. This change in the acidity of the mucus also changes its smell and taste. I suspect that this time your period is happening at the same time your vaginal mucus is shifting into the preovulation mode and getting thinner and more watery. It gets sweeter at this time too. Sometimes your period happens before the hormones of the next cycle have ramped up enough to cause this change but this month it sounds like both are happening at the same time. Decidual blood and implantation blood are usually light in flow and therefore browner and more tart smelling by the time they make it out -- since they would happen in the second half of the cycle when the pH makes the mucus more acidic and tart rather than sweet. Hope this helps!
Technically speaking, period blood has no odor. The blood can start to smell a bit when it mixes with air and bacteria starts to develop. To my knowledge, decidual or implantation bleeding is no different (be sure to check Molly’s answer to see what she has to say about that). What I do know is this – our bodies each have their own patterns and responses. You’ve noticed something different this month and are rightly asking about it. Typically, a foul odor is an indication that there might be an issue. Unless you’re using a new product such as a scented pad, I’m not sure what could cause a sweet smell. If there’s no product that could explain the smell, ask your healthcare professional about it.
When I’m dehydrated, my pee sometimes smells different than it normally does, but I unfortunately don’t have any experience with the smell you’re describing. I’ll defer to our health expert Molly for the meaning of down there smells and scents (including decidual and implantation blood). I’m super curious about the answer.