Why didn't she return sooner then, just because it would have been harder to explain the tattoos? Her mom was really worried for a week, and she surely knew she'd get fired. Yeah, she did get her job back but she didn't know that would happen, and she was working so that her mom could get out of debt.
Who knows why she didn't just come back to the instant after she left. Perhaps to make it slightly less obvious something was really fucking weird with her instantly having full sleeves out of nowhere. But she couldn't stay missing too long, she probably didn't want to worry her mom and the MC too much. So staying missing for a realistic timeframe for those sleeves to make sense was sort of off the table. So maybe she just came back to a point where the sleeves were questionable but hoped nobody would think too hard on the situation, instead of it being completely obvious something was up.
It's also possible that it's a similar type of time travel to what the MC uses, but they've found a way to change the ratio of time passing. Say, instead of 1 to 1 like the MC, it could be 1 day missing for every 4 in the other time period. If so, she couldn't choose when exactly to come back to, same as the MC.