There are a few things to pick up on here.
Introducing so they sniff each other through the cage is not how you introduce piggies.
You need to put them both/all in a neutral territory bonding pen for several hours so they can start to establish their relationship. After several hours, if all has gone well. you move them to their permanent cage together. Or then takes a further two weeks before the hierarchy is fully established.
I’m not quite clear from your post whether she is separates or in a cage with another piggy at present but do note that such a young baby cannot be alone. She must immediately be properly bonded and living in the cage with other piggies.
Separating her will cause a lot of stress and she must never be alone.
As you mention a herd, if you have other female with whom she will live in the same cage, do make sure you carry out the bonding properly with all piggies on neutral territory as Per the guide below.
This is the bonding guide which explains the full process
Bonding and Interaction: Illustrated Bonding Dynamics and Behaviours
Now to the health issue.
It is important that you weigh her. You need to know her baseline so that you can routinely weigh her to check she is eating enough hay and growing properly.
Lack of poop would indicate a lack of food intake 1-2 days ago, but she is likely stressed - it’s also why its important to know her weight
Do also remember that you cannot introduce lots of new fresh foods too quickly - you can cause a digestive upset if you do that. You need to only introduce one new food every week and allow her gut time to adjust
