Mowditate
That can happen. During meditation, there is less external distraction. Thoughts, memories, or feelings that are quickly overlaid by the next task in daily life can become more noticeable as a result.1
You do not have to push the sadness away or examine it especially intensely. You can begin by noticing what is present right now. How does the sadness feel in your body? Where do you feel it? If it feels all right to you, you can observe these sensations for a while. Afterwards, you can return your attention to your breath.
If the feeling becomes too strong, you may stop the meditation at any time. Open your eyes, move, and direct your attention back to your surroundings. Meditation is not a test of how much you can endure.
If intense sadness occurs frequently, lasts for a long time, or affects you outside meditation as well, it may be helpful to talk about it with someone you trust or a professional.