Kahlua was about 3-4 weeks as well. We tried the diapers and they rubbed too much so we actually ended up taking a trip to Target and finding some super soft and loose toddler shorts (technically it was a romper and I cut it and made it be toddler shorts). The toddler shorts worked great to keep the little droplets off the furniture and floor while not rubbing or irritating her at all. We would take it off whenever we let her run around in the back yard.
Kahlua continued to grow a bit after her first heat as well, a little bit of height and definitely more bulk and muscle.
I don't know if it is scientifically accurate but I always imagine that the lower energy and decreased appetite is because periods are as miserable for them as it is for me. I know I am pretty poopy and miserable whenever I'm in heat.