The Timeline makes perfect sense for Helaena

I don't know why everyone seems CONVINCED that the child cannot possibly be Aegon's just because Helaena's bump is still small.

It's a common misconception that all women start to "show" on/around one set time. Some women are not visibly pregnant until well into the second trimester. Some women start showing in the first trimester. Some women (one of whom I know personally) hardly ever show at all. All of which are perfectly normal.

Condal rapidly accelerated the timeline post Rook's Rest so Aegon would heal faster and not be out of commission as long. It's definitely been far less than 6 months since Aegon was burned.

Aegon's son and heir was murdered last season. Even though he and Helaena were both grieving, the reality is that they were in the middle of a war, and Aegon needed a new heir. It makes perfect sense that they might have been trying to conceive pre-Rook's Rest (or potentially even having sex out of shared sorrow, like Aegon I and Visenya did post-Rhaenys).