EDIT: Hey, OP here. Apparently we all needed to sit down and chat about why the whole running and lifting started, especially after I picked up my GF's father(I'll refer to him as F) with one hand and carried him back to the house like a logger just because I heard his wife(who I'll refer to as M) calling us back in for dinner. After we all ate, I got up for more running, lifting, and whatever else was needed, but F stopped me. Instead, he, M, and my GF(who I'll now refer to in this story as G) went over to the living room, inviting me to sit with them. After I did, F took a deep breath, looked at me, and told me to discuss why exactly I was feeling small and weak with informal orc greetings.

Turns out, it was a mix of culture shock, which they all expected, and both insecurity and stress, which G was most concerned about. Both M and F understood what I was going through, but they were taken aback when I told them about the root of my insecurities. It was my ex, a half-elf whom I'll call K. Turns out, G had dated K in the past, but they broke up due to some argument neither of them wanted to talk about. Turns out, this sparked something in K, because when I met her, she was as sweet as sugar, yet I soon found out that she was a massive racist.

Turns out I was starting to believe what she said about me once I broke up with her. Eventually, I managed to get the stress out with the running, lifting, and chores, yet M and F both thought they overreacted a bit. I already reconciled with G, so that went well. Unfortunately, I accidentally frightened them a bit when I deadlifted all 3 of them at once. I'll definitely have to adjust to this.