For your question, it can be inferred that your paper was submitted to one of the research journals published by the American Physical Society (most likely one of the Physical Review journals). So what I am going to say here will be, in part, based on my specific knowledge of the APS journals. However, most of this answer could be applied to a much more general situation.
In short, this is perfectly normal. Most of the time, manuscripts move through the early stages of the submission process more quickly; however, there is nothing especially unusual about it taking a few weeks longer for the status of the paper to change. A modest delay is nothing to worry about and can occur for totally innocuous reasons. For example, I have had first-hand experience with submissions to APS journals that appeared to be briefly held up for all the following reasons:
- There was discussion among the editorial staff of whether the manuscript was more appropriate for a different Physical Review journal.
- The editors were having unexpected problems getting one of the figures to appear correctly.
- One of the editors was out sick for a week.
- Someone at the editorial office forgot to change the manuscript's status in their system.
If I were you, I would not be worried at this point at all.